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PERSPECTIVE
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Anion-exchange membranes in electrochemical
energy systems†
Cite this: Energy Environ. Sci., 2014, 7,
3135
John R. Varcoe,*a Plamen Atanassov,b Dario R. Dekel,c Andrew M. Herring,d
Michael A. Hickner,e Paul. A. Kohl,f Anthony R. Kucernak,g William E. Mustain,h
Kitty Nijmeijer,i Keith Scott,j Tongwen Xuk and Lin Zhuangl
This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells,
electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g.
microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art,
Received 27th April 2014
Accepted 4th August 2014
technological and scientific limitations, and the future challenges (research priorities) related to the use
of anion-exchange membranes in these energy technologies. All the references that the authors
DOI: 10.1039/c4ee01303d
deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are
www.rsc.org/ees
included.
Broader context
Many electrochemical devices utilise ion-exchange membranes. Many systems such as fuel cells, electrolysers and redox ow batteries have traditionally used
proton-/cation-exchange membranes (that conduct positive charged ions such as H+ or Na+). Prior wisdom has led to the general perception that anion-exchange
membranes (that conduct negatively charged ions) have too low conductivities and chemical stabilities (especially in high pH systems) for application in such
technologies. However, over the last decade or so, developments have highlighted that these are not always signicant problems and that anion-exchange
membranes can have OH conductivities that are approaching the levels of H+ conductivity observed in low pH proton-exchange membrane equivalents. This
article reviews the key literature and thinking related to the use of anion-exchange membranes in a wide range of electrochemical and bioelectrochemical
systems that utilise the full range of low to high pH environments.
Preamble
a
Department of Chemistry, University of Surrey, Guildford, GU2 7XH, UK. E-mail: j.
varcoe@surrey.ac.uk; Tel: +44 (0)1483 686838
b
Department of Chemical and Nuclear Engineering, University of New Mexico,
Albuquerque, NM, 87131-0001, USA
c
CellEra, Caesarea Business and Industrial Park, North Park, Bldg. 28, POBox 3173,
Caesarea, 30889, Israel
d
Department of Chemical and Biological Engineering, Colorado School of Mines,
Golden, CO, 80401, USA
e
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, PA, 16802, USA
f
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology,
Atlanta, GA 30332-0100, USA
There is an increasing worldwide interest in the use of anionexchange membranes (including in the alkaline anion forms),
in electrochemical energy conversion and storage systems. This
perspective stems from the “Anion-exchange membranes for
energy generation technologies” workshop (University of
Surrey, Guildford, UK, July 2013), involving leading researchers
in the eld,1 that focussed on the use of AEMs in alkaline
polymer electrolyte fuel cells (APEFCs),2 alkaline polymer electrolyte electrolysers (APEE),3 redox ow batteries (RFB),4 reverse
electrodialysis (RED) cells,5 and bioelectrochemical systems
including microbial fuel cells (MFCs)6 and enzymatic fuel cells.7
g
Department of Chemistry, Imperial College London, South Kensington, London, SW7
2AZ, UK
h
Department of Chemical & Biomolecular Engineering, University of Connecticut,
Storrs, CT, 06269-3222, USA
i
Membrane Science and Technology, University of Twente, MESA+ Institute for
Nanotechnology, P.O. Box 217, 7500 AE, Enschede, The Netherlands
j
School of Chemical Engineering and Advanced Materials, Faculty of Science,
Agriculture and Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
k
School of Chemistry and Material Science, USTC-Yongjia Membrane Center,
University of Science and Technology of China, Hefei, 230026 P. R. China
l
Department of Chemistry, Wuhan University, Wuhan, 430072, P. R. China
† Electronic supplementary information (ESI) available: This gives the
biographies for all authors of this perspective. See DOI: 10.1039/c4ee01303d
This journal is © The Royal Society of Chemistry 2014
Conventions used in this perspective article
In this article the following terminology is dened:
AEM is used to designate anion-exchange membranes in
non-alkaline anion forms (e.g. containing Cl anions);
AAEM used to designate anion-exchange membranes
containing alkaline anions (i.e. OH, CO32 and HCO3);
HEM is used to designate hydroxide-exchange membranes
and should only be used where the AAEMs are totally separated
from air (CO2) and are exclusively in the OH form (with no
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traces of other alkaline anions such as CO32); this is not the
case in most of the technologies discussed in the article (a
possible exception being APEEs);
AEI is used to designate an anion-exchange ionomer which
are anion-exchange polymer electrolytes in either solution or
dispersion form: i.e. anion-exchange analogues to the protonexchange ionomers (e.g. Naon® D-52x series) used in protonexchange membrane fuel cells (PEMFCs). AEIs are used as
polymer binders to introduce anion conductivity in the electrodes (catalyst layers).
CEM is used to designate cation-exchange membranes in
non-acidic form (e.g. containing Na+ cations);
PEM is used to designate proton-exchange membranes (i.e.
CEMs specically in the acidic H+ cation form);
Perspective
IEM is used to designate a generic ion-exchange membrane
(can be either CEM or AEM).
Note that in this review, all electrode potentials (E) are given
as reduction potentials even if a reaction is written as an
oxidation.
AEMs and AEIs for electrochemical
systems
Summary of AEM chemistries used in such systems
AEMs and AEIs are polymer electrolytes that conduct anions,
such as OH and Cl, as they contain positively charged
[cationic] groups (typically) bound covalently to a polymer
backbone. These cationic functional groups can be bound
Professor John Varcoe (Department of Chemistry, University of
Surrey, UK) obtained both his
1st class BSc Chemistry degree
(1995) and his Materials Chemistry PhD (1999) at the University of Exeter (UK). He was a
postdoctoral researcher at the
University of Surrey (1999–
2006) before appointment as
Lecturer (2006), Reader (2011)
and Professor (2013). He is
recipient of an UK EPSRC Leadership Fellowship (2010). His research interests are focused on
polymer electrolytes for clean energy and water systems: more
specically, the development of chemically stable, conductive
anion-exchange polymer electrolytes. He is also involved in the
University's efforts on biological fuel cells.
Professor
Michael
Hickner
(Associate Professor, Department of Materials Science and
Engineering, Pennsylvania State
University, USA) focuses his
research on the relationships
between chemical composition
and materials performance in
functional polymers to address
needs in new energy and water
purication applications. His
research group has ongoing
projects in polymer synthesis,
fuel cells, batteries, water treatment membranes, and organic
electronic materials. His work has been recognized by a Presidential Early Career Award for Scientists and Engineers from
President Obama (2009). He has co-authored seven US and
international patents and over 100 peer-reviewed publications
with >5400 citations.
Dr Dario Dekel (Co-Founder and
VP for R&D and Engineering,
CellEra, Israel) received his
MBA, MSc and PhD in Chemical
Engineering from the Technion,
Israel Institute of Technology.
He was the chief scientist and
top manager at Rafael Advanced
Defense Systems, Israel, where
he led the world's second largest
Thermal Battery Plant. He le
Rafael in 2007 to co-found CellEra, leading today a selected
group of 14 scientists and engineers, developing the novel Alkaline
Membrane Fuel Cell technology. He currently holds $3M government research grants from Israel, USA and Europe. Dr Dekel holds
14 battery and fuel cell patents.
Professor Paul Kohl (Hercules
Inc./Thomas L. Gossage Chair,
Regents' Professor, Georgia
Institute of Technology, USA)
received a Chemistry PhD
(University of Texas, 1978). He
was then involved in new chemical processes for silicon and
compound
semiconductor
devices at AT&T Bell Laboratories (1978–89). In 1989, he
joined Georgia Tech.'s School of
Chemical and Biomolecular
Engineering. His research includes ionic conducting polymers, high
energy density batteries, and new materials and processes for
advanced interconnects for integrated circuits. He has 250 papers,
is past Editor of JES and ESSL, past Director MARCO Interconnect
Focus Center, and President of the Electrochemical Society
(2014–15).
3136 | Energy Environ. Sci., 2014, 7, 3135–3191
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Perspective
either via extended side chains (alkyl or aromatic types of
varying lengths) or directly onto the backbone (oen via CH2
bridges); they can even be an integral part of the backbone.
The most common, technologically relevant backbones are:
poly(arylene ethers) of various chemistries8 such as polysulfones [including cardo, phthalazinone, uorenyl, and
organic–inorganic hybrid types],9 poly(ether ketones),10,11
poly(ether imides),12 poly(ether oxadiazoles),13 and poly(phenylene oxides) [PPO];14 polyphenylenes,15 peruorinated
types,16,17 polybenzimidazole (PBI) types including where the
cationic groups are an intrinsic part of the polymer backbones,18 poly(epichlorohydrins) [PECH],19 unsaturated polypropylene20 and polyethylene21 types [including those formed
using ring opening metathesis polymerisation (ROMP)],22 those
based on polystyrene and poly(vinylbenzyl chloride),23 polyphosphazenes,24 radiation-graed types,25 those synthesised
using plasma techniques,26 pore-lled types,27 electrospun bre
types,28 PTFE-reinforced types,26g,29 and those based on poly(vinyl alcohol) [PVA].19a,30
The cationic head-group chemistries that have been studied
(Scheme 1), most of which involve N-based groups, include:
(a) Quaternary ammoniums (QA) such as benzyltrialkylammoniums [benzyltrimethylammonium will be treated as
the benchmark chemistry throughout this report],2,31 alkyl-bound
(benzene-ring-free) QAs,21a,b and QAs based on bicyclic ammonium systems synthesised using 1,4-diazabicyclo[2.2.2]octane
(DABCO) and 1-azabicyclo[2.2.2]octane (quinuclidine, ABCO)
(to yield 4-aza-1-azoniabicyclo[2.2.2]octane,14b,25f,h,27g,32 and 1azoniabicyclo[2.2.2]octane {quinuclidinium}19c,33 functional
groups, respectively);
(b)
Heterocyclic
systems
including
imidazolium,10a,13,23a,25g,31,34 benzimidazoliums,35 PBI systems where the
positive charges are on the backbone (with or without positive
charges on the side-chains),18b,d,f,h,36 and pyridinium types (can
only be used in electrochemical systems that do not involve
high pH environments);26h,i,30i,37
Professor Tongwen Xu (University of Science and Technology of
China) received his BSc (1989)
and MSc (1992) from Hefei
University of Technology and his
Chemical Engineering PhD
(1995) from Tianjin University.
He then studied polymer science
at Nankai University (1997). He
was visiting scientist at University of Tokyo (2000), Tokyo
Institute of Technology (2001)
and Gwangju Institute of Science
and Technology (Brain Pool Program Korea award recipient). He
has received a “New Century Excellent Talent” (2004) and an
“Outstanding Youth Foundation” (2010) Chinese awards. His
research interests cover membranes and related processes,
particularly ion exchange membranes and controlled release.
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(c) Guanidinium systems;16c,38
(d) P-based systems types including stabilised phosphoniums [e.g. tris(2,4,6-trimethoxyphenyl)phosphonium]11,14d,32a,39
and P–N systems such as phosphatranium16d and tetrakis(dialkylamino)phosphonium40 systems;
(e) Sulfonium types;41
(f) Metal-based systems where an attraction is the ability to
have multiple positive charges per cationic group.42
General comments on the characterisation of AEMs
Given that OH forms of AAEMs quickly convert to the less
conductive CO32 and even less conductive HCO3 forms when
exposed to air (containing CO2 – see eqn (1) and (2)), even for
very short periods of time,25d,43 it is essential that CO2 is totally
excluded from experiments that are investigating the properties
of AAEMs in the OH forms. This includes the determination of
water uptakes, dimensional swelling on hydration, long-term
stabilities, and conductivities [see specic comments in the
below sections].
OH + CO2 # HCO3
(1)
OH + HCO3 # CO32 + H2O
(2)
Additionally, when converting an AEM or AEI into a single
anion form, it is vital to ensure complete ion-exchange. An IEM
cannot be fully exchanged to the desired single ion form aer
only 1 immersion in a solution containing the target ion, even
if a concentrated solution containing excess target ion is used:
the use of only 1 immersion will leave a small amount of the
original ion(s) in the material (ion-exchange involves partition
equilibria). IEMs must be ion-exchange by immersion in
multiple (at least 3) consecutive fresh replacements of the
solution containing an excess of the desired ion. Traces of the
original (or other contaminant) ions can have implications
regarding the properties being measured.44
Professor Lin Zhuang (Department of Chemistry, Wuhan
University, China) earned his
electrochemistry PhD (1998) at
Wuhan University. He was then
promoted to lecturer, associate
professor (2001) and full
professor (2003). He was a
visiting scientist at Cornell
(2004–05) and is an adjunct
professor at Xiamen University.
He is an editorial board member
of Science China: Chemistry,
Acta Chimica Sinica, and Journal of Electrochemistry. He was
recipient of a National Science Fund for Distinguished Young
Scholars. He was vice-chair of the physical electrochemical division of the International Society of Electrochemistry (2011–12) and
China section chair of the Electrochemical Society (2010–11).
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Perspective
Scheme 1 Commonly encountered AAEM/AEI cationic head-groups (those containing N–H and P–H bonds are omitted): A ¼ benzyltrialkylammonium (the benchmark benzyltrimethylammonium is where R, R0 , and R00 are methyl groups); B ¼ alkyl-side-chain (benzene-free)
quaternary ammonium (QA) and crosslinking diammonium groups (where the link chain is >C4 in length [preferably >C6 in length]); C ¼ DABCObased QA groups (more stable when only 1 N atom is quaternised [crosslinked systems where both Ns are quaternised are also of interest but are
less stable in alkali]); D ¼ quinuclidinium-based QA groups; E ¼ imidazolium groups (where R ¼ Me or H and R0 , R00 ¼ alkyl or aryl groups [not H]);
F ¼ pyridinium groups; G ¼ pentamethylguanidinium groups; H ¼ alkali stabilised quaternary phosphonium groups; I ¼ P–N systems (where X ¼
–SO2– or –NR0 – groups and where R ¼ alkyl, aryl, or unsaturated cyclic systems); and J is an exemplar metal containing cationic group.
One of the main properties that must be reported for each
AEM/AEI produced is the ion-exchange capacity (IEC), which is
the number of functional groups (molar equivalents, eq.) per
unit mass of polymer. In the rst instance, it is highly recommended that the IEC of the Cl form of the AEM being studied is
measured (the form typically produced on initial synthesis).19e,31,45
This is so that the AEMs have not been exposed to either acids or
bases that may cause high and low pH-derived degradations
(even if such degradations are only slight) and to avoid signicant CO2-derived interferences: both acid and bases are
required for the use of the classical back-titration method of
determining IECs of AAEMs.46 Additionally when using Cl
based titrations, methods are available to measure the total
exchange capacities, quaternary-only-IEC and non-quaternary
(e.g. tertiary) exchange capacities.47 These techniques will be
useful for AAEM degradation studies where QA groups may
degrade into polymer-bound non-QA groups (such as tertiary
amine groups). However, there can be discrepancies between
IECs derived from titration experiments and other techniques
such as those that use ion-selective electrodes or spectrophotometers.48 NMR data can also be used to determine IECs with
soluble AEMs and AEIs.49,50
Perceived problems with the use of AAEMs
The two main perceived disadvantages of AAEMs are low
stabilities in OH form (especially when the AAEMs are not fully
hydrated)51 and low OH conductivities (compared to the H+
conductivity of PEMs, especially [again] when the AAEMs are
3138 | Energy Environ. Sci., 2014, 7, 3135–3191
not fully hydrated).19d,52,53 The former is going to be challenging
problem to solve if the electrochemical system in question
requires the conduction of OH anions (i.e. a strong nucleophile) as the polymer electrolytes contain positively charged
cationic groups (i.e. good leaving groups!). Conductivity issues
are not insurmountable with improved material and cell
designs. While conductivities of ca. 101 S cm1 are needed for
high current density cell outputs, operating electrochemical
devices with membranes that have intrinsic conductivities of
the order of 5 102 S cm1 is not out of the question.
Conductivities of 102 S cm1 may, however, be too low for
many applications.
The alkali stabilities of AAEMs. A primary concern with the
use of AAEMs/AEI in electrochemical devices such as APEFCs and
APEEs is their stabilities (especially of the cationic head-groups)
in strongly alkaline environments (e.g. in the presence of nucleophilic OH ions). This alkali stability issue has dominated
discussions such that radical-derived degradations (e.g. from the
presence of highly destructive species such as OH_ radicals that
originate from peroxy species generated from the n ¼ 2e oxygen
reduction reaction [ORR]) have only been considered in a small
number of reports.23a,54 This is a major long-term degradation
issue with PEMs in PEMFCs.55 The perception has been that
AAEM/AEI degradation via attack by OH anions is so severe over
short timeframes that radical-derived degradation cannot be
studied until alkali stable AAEMs/AEIs have been developed. This
assumption needs to be challenged especially as AAEMs/AEIs
tend to be hydrocarbon or aromatic based, which have poor
peroxide and oxidation stabilities.
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It is apparent in Scheme 2 that even simple benzyltrimethylammonium cationic functional groups (the most
commonly encountered, Scheme 1A) can undergo a number of
degradation processes in the presence of OH nucleophiles.
The
main
degradation
mechanism
for
benzyltrimethylammonium groups is via direct nucleophilic substitution (displacement). The formation of intermediate ylides
(>C–N+R3) have been detected via deuterium scrambling
experiments and these can potentially lead to Sommelet–
Hauser and Stevens rearrangements;51c however, such ylidederived mechanisms rarely end in a degradation event.51a Hofmann elimination reactions cannot occur with benzyltrimethylammonium as there are no b-Hs present; this is not the
case with benzyltriethylammonium groups,51b,56 which contain
b-Hs [even though the R–N+(CH2CH3)3 OH groups may be
more dissociated than R–N+(CH3)3 OH groups]. As an aside,
neopentyltrimethylammonium groups (on model small
compounds, i.e. not polymer bound) contain a long alkyl chain
but with no b-Hs: Hofmann elimination cannot occur, but the
degradation of this cationic group appears to be even more
complex with unidentied reaction products detected.51b
Historically, due to concerns about facile Hofmann elimination reactions, QAs bound to longer alkyl chains were
considered to be less stable than those bound to aromatic
Energy & Environmental Science
groups via –CH2– bridges.56 However, more recent evidence
suggests that this may not be the case and that QA groups that
are tethered (or crosslinked) with N-bound alkyl chains that are
>4 carbon atoms long (C4, see Scheme 1B) can have surprisingly
good stabilities in alkali.13a,47a,57,58 A hypothesis is that the high
electron density around the b-Hs in longer alkyl chains can
inhibit Hofmann elimination reactions47a and that steric
shielding in the b-positions may also play a role in the
surprising stability imparted by longer alkyl chains.57g
The search for alkali stable AAEMs/AEIs is the primary driver
for the study of alternative cationic head-group chemistries. An
alternative QA system is where DABCO is used as the quaternisation agent (Scheme 1C). This system contains b-Hs but due
to the rigid cage structure, the b-Hs and the N atoms do not
form the anti-periplanar conrmation required for facile Hofmann Elimination to occur (Scheme 3).19e,59,60 It is suspected
that AAEMs/AEIs containing 4-aza-1-azoniabicyclo[2.2.2]octane
groups, where only 1 N of the DABCO reactant is quaternised,
are more stable than R–N+Me3 analogues.32e,60 However, it is not
easy to produce AAEMs/AEIs where only 1 N of the DABCO
reagent reacts (although low temperatures may be helpful in
this respect).59 The tendency is for DABCO to react via both N
atoms forming crosslinks, which will produce materials
with low alkali stabilities.32e This has led to the recent interest
Degradation pathways for the reaction of OH nucleophiles with benzyltrimethylammonium cationic (anion-exchange) groups.51
The inset [dashed box] shows the additional Hofmann Elimination degradation mechanism that can occur with alkyl-bound QA groups (that
possess b-H atoms).
Scheme 2
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Scheme 3
in
quinuclidinium-(1-azoniabicyclo[2.2.2]octane)
systems
(Scheme 1D), which is a DABCO analogue containing only 1 N
atom.19c,33 However, quinuclidine is much harder to synthesis
than DABCO (harder to “close the cage”) and this is reected in
the price: quinuclidine ¼ US$775 for 10 g vs. DABCO ¼ US$34
for 25 g (laboratory scale prices, not bulk commodity prices).61
Also, quinuclidine is highly toxic (e.g. Hazard statement H310 –
Fatal in contact with skin).62 This must be taken into account if
quinuclidinium-containing AAEMs/AEIs degrade and release
any traces of quinuclidine.
Systems involving >1 N atoms and “resonance stabilisation”
have been evaluated with the desire of developing alkali stable
and conductive AAEMs/AEIs. Firstly, non-heterocyclic pentamethylguanidinium systems (made using 1,1,2,3,3-pentamethylguanidine) have been reported,38 including peruorinated
AAEM examples.16c However, more recent studies suggest that
this system may not be as alkali stable as originally reported.38e
Polymer bound benzyltetramethylguanidinium (i.e. addition of
a benzyl substituent) is reported to be more alkali stable than
polymer bound pentamethylguanidinium groups.63 However,
other reports indicate that guanidiniums bound to the polymer
backbone via phenyl groups may be more stable than those
bound via benzyl groups and peruorosulfone groups.16c,64
These prior reports indicate that new degradation pathways (cf.
QA benchmarks) are available with this cationic head-group.
The other multiple N atom system that has been extensively
reported is the heterocyclic imidazolium system (Scheme 4).
This includes where imidazolium groups have been used to
introduce covalent crosslinking into the system.65 Imidazolium
systems where R2, R4, and R5 are all H atoms are unstable to
Scheme 4 Imidazolium-based cationic head-group chemistry: A ¼
benzyl-bound imidazolium groups; B ¼ alkyl-bound imidazolium
groups; and C the alkali stabilised imidazolium group reported by Yan
et al.34b
3140 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
alkali.31,34c,34f,66 Polymer bound imidazolium groups with R2 ¼ H
can degrade via imidazolium ring-opening in the presence of
OH ions.34l,67 Replacement of the protons at the C2 position
(e.g. R2 ¼ Me or butyl group) increases the stability of the imidazolium group.34c,d,68 Different substituents at the N3 position
(R3 ¼ butyl, isopropyl, amongst others) can also affect the alkali
stability of the imidazolium group:68a,69 systems where R3 ¼
isopropyl or R2 ¼ R3 ¼ butyl groups are reported to be more
stable options.
Yan et al. has recently reported an alkali stable PPO-bound
imidazolium group [made using 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl)imidazole] that contains no C–H bonds on the
imidazolium ring and no C2 methyl group (Scheme 4C).34b This
sterically bulky functional group was at least as stable as QA
benchmarks. This claimed alkali stability is also backed up by
DFT measurements in another recent study by Long and Pivovar, which suggests that similar C2-substituted imidazoliums
will have superior alkali stabilities.70 Alkyl-2,3-dimethylimidazolium groups (R2 and R3 ¼ Me) that are bound via long alkyl
chains34k may also be more alkali stable than benzyl-bound
analogues: the latter undergo facile removal of the imidazolium
rings via nucleophilic displacement reactions34c (as well as
degradation via imidazolium ring-opening).
However, contrary to the above, a study of small molecule
imidazolium species by Price et al. suggests that adding steric
hindrance at the C2 position is the least effective strategy;71 this
study reports that 1,2,3-trimethylimidazolium cations appear to
be particularly stable and this matches our experience in that
the 1,2,3,4,5-pentamethylimidazolium cation appears to be
reasonably stable in alkali. Furthermore, it has been reported
that as you add more bulky cations, such as the 1,4,5-trimethyl2-(2,4,6-trimethoxyphenyl)imidazole highlighted above, the
anion transport switches from Arrenhius-type to Vogel–Tamman–Fulcher-type behaviour (i.e. the anions become less
dissociated).72 Other studies that have looked into the alkali
stability of various imidazolium-based ionic liquids, however,
report that all 1,3-dialkylimidazolium protons (R2,R4, and R5 ¼
H) can undergo deuterium exchange (i.e. represent alkali
stability weak spots) and that even C2 methyl groups (R2 ¼
–CH3) can undergo deprotonation in base.73 Further fundamental research into these and related systems is clearly still
warranted.
The stabilised PBI system poly[2,20 -(m-mesitylene)-5,50 bis(N,N0 -dimethylbenzimidazolium)], where the cationic group
is part of the polymer backbone, has recently been reported with
promising alkali stabilities.18d This research has led to the
development of other PBI-type ionenes that contain sterically
protected C2 groups and are soluble in aqueous alcohols but
insoluble in pure water;74 they are reported to have “unprecedented” hydroxide stabilities.
Regarding P-based systems, phosphonium AAEMs/AEIs are
also common in the recent literature. Yan et al. rst reported an
alkali-stabilised polymer-bound phosphonium system made
using tris(2,4,6-trimethoxyphenyl)phosphine as the quaternising agent (Scheme 1H) where the additional methoxy groups are
electron donating and provide additional steric hinderance.11,14d,32a,39,66 This stabilisation is important as simple
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trialkylphosphonium and triphenylphosphonium analogues
(e.g. small molecule benzyltriphenylphosphonium cations) will
degrade in aqueous OH solutions at room temperature in only
a few hours; the thermodyanamic driving force being the
formation of phosphine oxide via the Cahours–Hofmann reaction (especially in the presence of organics).73,75 However, recent
spectroscopic studies suggest this bulky (high molecular
weight) head-group still degrades in alkali.66 Initial results with
the P–N tetrakis(dialkylamino)phosphonium system [poly-(Me)
N–P+(–N(Me)Cy)3 where Cy ¼ cyclohexane] rst reported by
Coates et al. suggests that this type of cationic head-group
chemistry may be stable to alkali40 as indicated by early reports
on small molecule studies.76
Prior thinking was that the alkali stability of the cationic
head-groups could be treated separately to the chemical
stability of the polymeric backbone (e.g. once an alkali stable
head-group is found it can be attached to whatever polymer
backbone is required and the polymer backbone and headgroup will remain alkali stable). However, recent results suggest
a much more complex situation with a symbiosis between the
stability of the head-groups and the polymer backbone. For
example, polysulfone itself is stable when exposed to aqueous
alkali but is destabilised and degrades in high pH environments
when QA groups are attached to the polymer backbone (via
–CH2– linkages): the polymer backbone becomes partially
hydrophilic, allowing close approach of the OH anions.77 The
electron withdrawing sulfone linkage has a profound negative
inuence on the stabilities of the resulting AAEMs.78 The
hydrophobicity of unfunctionalised plastics lends signicant
resistance to alkali and it therefore stands to reason that more
OH uptake into the polymer structure will induce greater
degradation. The degradation of AAEM backbones in alkali have
been observed for other systems.15b,33b,34f The alkali stabilities of
the following backbones containing pendent trimethylammonium cationic groups appear to decrease in the following order
(Scheme 5): polystyrene > PPO > polysulfone (and all were less
stable than the model small molecule p-methylbenzyltrimethylammonium).77 Note that with polysulfone AAEMs,
strategies are now being developed to move the QA group away
from the polysulfone backbone, where an additional benzyl
group is located between the QA group and the backbone.79
Backbone stability may also be enhanced if phase segregated
systems are developed (see later). The development of cationic
side-chains containing multiple positive charges may help due
to the ability to widely disperse the cationic side chains along
the polymer backbone (charged groups placed further apart
from each other without changing the IEC).80 Therefore, when
evaluating the alkali stabilities of new AAEMs/AEIs, the head-group
and backbone must be evaluated together in combination.
Another problem with evaluating the ex situ alkali stabilities
of different AAEMs/AEIs with different chemistries is the broad
range of different methodologies used throughout the literature. A common approach is to evaluate the change of ionic
conductivity of the materials with increasing immersion times
in aqueous alkali. This can be a useful measure of alkali
stability but there is a risk of false positives: if the degraded
membranes exhibit ionic conductivity, then the original AAEM/
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Energy & Environmental Science
Scheme 5 The relative alkali stability of various polymer backbones
when containing pendent trimethylammonium groups.77
AEI may appear more alkali stable than it really is. A more useful
measure of alkali stability is the measurement of IEC with
increasing immersion times in aqueous alkali. This will be even
more useful if changes in both quaternary IEC and nonquaternary exchange capacities are studied (see earlier discussion on IECs by titration). However, the authors recommend
that such secondary measurements of alkali stability (changes
in ionic conductivity and IEC) are always supplemented with
multiple spectroscopic measurements (e.g. NMR,30f,33b,34c,66,69,77,81
IR,34k,82 and Raman31,34c).
Clearly as more alkali stable AAEMs/AEIs are developed, ex
situ accelerated test protocols must be developed, i.e. immersion in concentrated alkali at high temperatures [e.g. aqueous
KOH (6 mol dm3) at 80–90 C] with/without addition of peroxy/
radical-based degradation agents. However, it must be kept in
mind that if the aqueous alkali is too concentrated, viscosity
effects may come into play and interfere with the stability
measurements (e.g. diffusion of OH nucleophiles towards the
cationic groups is retarded). Data from accelerated degradation
studies conducted inside NMR spectrometers (with soluble
AAEMs/AEIs) will allow the simple and quick production of
useful stability data (including an idea of the degradation
mechanism that is operating).77 All of these ex situ stability
measurements must be validated/benchmarked against in situ
real-world and accelerated durability tests (in the spirit of DOE
protocols for PEMFCs).55
It should also be kept in mind that the AAEMs/AEIs inside
APEFCs are in an environment in the absence of excess metal
(e.g. Na+ or K+) hydroxide species. Therefore, ex situ stability
data at high temperatures with the AAEMs/AEIs in OH forms
in the absence of additional/excess NaOH or KOH species is also
useful (e.g. an OH form AAEM submerged in deionised water
at 90 C). The challenge here will be to ensure the AAEMs/AEIs
remain in the OH form (i.e. CO2 is totally excluded from all
stages of the experiments [not easy to achieve]) as the AAEMs/
AEIs will be more stable in the HCO3 and CO32 anion forms.
Warder titration methods43b will be useful as these measure the
relative contents of OH, CO32 and HCO3 anions in the
polymer electrolyte materials with time (example data given in
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Fig. 1 for an AAEM [originally in the OH form] that is exposed
to air); control experiments can be run alongside the degradation experiments where additional AAEMs/AEIs samples, originally in the OH forms and kept in the same environment as the
primary degradation samples, are monitored for a reduction in
OH content and an increase in CO32/HCO3 content.
However, despite all of the studies into the different chemistries, the benzyltrimethylammonium hydroxide group may be
stable enough for some applications (even those that contain
alkali environments) as long as the benzyltrimethylammonium
head-groups are kept fully hydrated (the OH anion is less
nucleophilic when it possesses a full hydration shell).51d This is
more true for the use of this cationic group in the AAEMs but
less true for use in the AEIs that are exposed to gas ows (much
more difficult to maintain the AEIs in the fully hydrated state).
Tailoring the hydrophobicity of the cationic group's environment may well have an impact.56 The challenge for applications
such as APEFCs, where it is difficult to keep the polymer electrolyte components fully hydrated (unlike in APEEs), is to
develop AAEMs and (especially) AEIs that are stable (and
conductive) in the presence of OH when less than fully
hydrated.
AAEM conductivities. The most commonly cited reasons for
the lower conductivities of AAEMs/HEMs vs. PEMs are:
(a) The lower mobility of OH (and HCO3/CO32) vs. H+ (see
Table 1);84
(b) The lower levels of dissociation of the ammonium
hydroxide groups (cf. the highly acidic R–SO3H groups in PEMs).
With regards to (a) above, the intrinsically lower mobilities
are traditionally offset by using AEMs with higher IECs
compared to PEMs because ionic conductivity f ion mobility ion concentration. AEMs typically possess IECs much higher
than 1.1 meq. g1 (cf. Naon®-11x series of PEMs ¼ 0.91–0.98
meq. g1)85 apart from of the state-of-the-art phase segregated
systems discussed in detail later. This can lead to problems with
high water uptakes and dimensional swelling (hydrated vs.
dehydrated) and this leads to AEMs with lower mechanical
Perspective
Table 1
Select ion mobilities (m) at infinite dilution in H2O at 298.15 K
Ion
Mobility (m)/108
m2 s1 V1
Relative mobilitya
(relative to K+)
Ref.
H+
OH
CO32
HCO3
Na+
Cl
K+
36.23
20.64
7.46
4.61
5.19
7.91
7.62
4.75
2.71
0.98
0.60
0.68
1.04
1.00
87 and 88
87 and 88
87 and 88
87
88
88
88
a
Calculated from the mobility data to the le and in general agreement
with the relative mobility data presented in ref. 89.
strengths and a difficulty in maintaining the in situ integrity of
membrane electrode assemblies (MEAs) containing AEMs
(especially for APEFCs that use gas feeds [the MEAs are not in
continuous contact with aqueous solutions/water]).86
Regarding (b) above and “lower levels of dissociation”, it is
oen stated that “trimethylamine is a weak base” as it has a pKb
value (in aqueous solutions) of only ca. 4.2 (pKa [z 14 pKb] ¼
9.8 for the conjugate trimethylammonium [Me3N+H] cation):88
Kb ¼
aðNMe3 Hþ Þ aðOH Þ
aðNMe3 Þ
Ka ¼
aðHþ Þ aðNMe3 Þ
aðNMe3 Hþ Þ
for NMe3 Hþ #Hþ þ NMe3
3142 | Energy Environ. Sci., 2014, 7, 3135–3191
(4)
where pKa ¼ log(Ka), pKb ¼ log(Kb), Kb is the relevant base
dissociation constant, Ka is the dissociation constant for the
conjugate acid trimethylammonium (¼ 1.6 1010), and a(X)
are the activities (activity coefficient corrected concentrations)
of the various species in solution. However these are not the
relevant equilibria to consider for QA hydroxides such as benzyltrimethylammonium hydroxide: these contain no N–H
bonds! Take the simplest exemplar tetramethylammonium
hydroxide (which has never been isolated in anhydrous form):
NMe4OH is a very strong base (used industrially for the anisotropic etching of silicon) and has a conjugate acid pKa > 13.90
Similarly, benzyltrimethylammonium hydroxide (also known as
Triton B) is also a strong base and has been used as the catalyst
in various base catalysed organic reactions.91 The relevant
equilibrium is more analogous to aqueous alkali metal
hydroxides (e.g. aqueous KOH):
RNMe3OH # RNMe3+ + OH
Fig. 1 IECs (quaternary) of different alkali anions for a benzyltrimethylammonium-type ETFE-radiation-grafted AAEM (80 mm thick)
that was initially exchanged to the OH form and then directly exposed
to air. IECs determined using Warder titration methods.43b,83 Error bars
are sample standard deviations (n ¼ 3 repeats).
(3)
for NMe3 þ H2 O#NMe3 Hþ þ OH
(5)
Indeed, AEMs in the OH (and F) forms appear to be
completely dissociated at high hydration levels (in CO2-free
conditions) unlike AEMs in the I, Cl, Br, and HCO3 forms
and the OH ions conduct mainly via structure diffusion
(approaching half the conductivities of H+ in PEMs).92 Therefore, concerns over the low levels of dissociation of OH for N–H
bond free QA hydroxide groups (in AAEMs) are generally
overstated.
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As the AAEMs quickly convert to less conductive CO32/
HCO3 forms when exposed to CO2 (i.e. air, recall Fig. 1),25d,43,93
it is essential that CO2 (air) is totally excluded from conductivity
determinations of OH form AAEMs. It is clear from the literature that this is rarely the case and that different laboratories
use different set-ups probably with different levels of CO2
exclusion. This creates problems with regard to inter-laboratory
comparisons of OH conductivities. Most groups are likely
underestimating the OH conductivities of their AAEMs/AEIs
due to [difficult to obtain] incomplete CO2 exclusion (i.e. they
are measuring the conductivities of mixed alkali [OH/HCO3/
CO32] anion forms). Therefore to aid inter-laboratory comparisons, we make the recommendation that HCO3 conductivities are
always reported for AAEMs/AEIs alongside conductivity data in
other anion forms, such as Cl and OH (the water uptake of the
material in each of the anion forms must also be measured to
understand the conductivity changes in the material). The rationale is the AAEMs remain predominantly in the HCO3 form in
the presence of air (CO2) and that the OH conductivities can be
estimated from the measured HCO3 conductivities.31,53
However, caution is required with such estimates as OH
conductivities for AAEMs containing benzyltrimethylammonium cations have been measured to be higher than the
size of the cation would normally indicate.93 There is also the
added complication with materials of a hydrophobic nature as
ion-exchange is oen incomplete and small amounts of
residual anions can have profound effects on the mobility of the
ion that you think you are studying.44
It should also be kept in mind that the conductivities of most
relevance to electrochemical devices are the through plane
conductivities as the ions move from one electrode to the other
through the thickness of the AAEM. The measurement of in-plane
conductivities (typically using 4-probe techniques) can sometimes
lead to an overestimation of the ionic conductivities (i.e. conductivities are oen anisotropic) with a bias towards the conductivities
across the surface layers of the membranes (sometimes the most
functionalised parts).94 However, we acknowledge that the
measurement of through-plane conductivities can be tricky (difficult to isolate the membrane resistance from the electrode interfacial resistance when the membrane thicknesses are smaller than
the dimensions of the electrodes) and that in-plane measurements
have their place as they are oen much more repeatable (and yield
results that are less likely to be misinterpreted).
In devices where the AEMs/AEIs are not in continuous
contact with liquid water (e.g. APEFCs) it is essential that they
can conduct at lower relative humidities (RH). This will be a big
challenge as the conductivities (and chemical stabilities) of
AAEMs drop off much more rapidly with RH than with
PEMFCs.19d,52 Hence, measurements in liquid water are not
always relevant because fuel cell developers want ionic
conductivities reported with the membranes in water vapour
(reviewers oen push that conductivity measurements where
the membranes are immersed in liquid water should be
reported). These are much harder to conduct especially when
the measurement of the OH conductivities of AAEMs/AEIs in
RH # 100% atmospheres is desired (the use of glove boxes with
CO2-free atmospheres are essential).15a
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Fundamental studies (including modelling studies) related
to anion conductivity, the effect of water contents and transport,
and the effect of CO2 on the properties of AAEMs have been
undertaken.19d,46a,95 These should continue in order to understand what is required to maintain high conductivities under
lower humidity environments (low water content per exchange
group) and the effect of the presence of CO2 on AAEM
conductivity (see APEFC section later). For fundamental studies,
it is oen useful to normalise conductivities to other factors
such as, water contents,50 IECs39b and mobilities.46a Additional
experiments such as the measurement of NMR T1 and T2
relaxation times for water in AAEMs can also be useful.49 Short
water relaxation times can lead to improved AAEM conductivity
(even with lower IECs) as they indicate more close interaction
between the water molecules and the solid polymer. Too high
water uptake (oen via excessively high IEC) can mean that
much of the H2O is inactive (not interacting with polymer) and
is actually diluting the conductive species (leading to a lowering
of the conductivity).
Various strategies have been proposed to enhance the
conductivities of AAEMs without employing excessively high
IECs and water uptakes (dimensional swelling). The development of phase-segregated AAEMs, containing hydrophobic
phases interspersed with hydrophilic ionic channels and clusters (à la Naon®), is rapidly becoming the de facto strategy for
developing the high conductivity AAEMs with low IECs and
water uptakes (see next section). It should be noted that phase
separation is not always essential for high AAEM/AEI conductivities.20 Covalent crosslinking is an alternative strategy, which
can additionally reduce gas crossover but may also lead to less
desirable attributes such as insolubility, reduced exibility and
embrittlement (leading to poor membrane processability), and
even a loss of conductivity (if it interferes with phase segregation).10d,13a,14b,d,25a,96 Other strategies include ionic crosslinking,10b maximising the van der Waals interactions (to
minimise swelling without the use of crosslinks),14d using 1,2,3triazoles to link the QA groups to the polymer backbone,97 and
enhancing the number of positive charges on the sidechains.42d,80
Phase segregated AEMs98
A realistic strategy to enhance the ionic conductivity of AAEMs
is to improve the effective mobility of OH rather than
increasing the IEC (to avoid excessive water uptakes and
dimensional swelling on hydration).2e As shown in Table 1, the
mobility of OH in dilute KOH solution is actually rather high
and is only inferior to that of H+ but much superior to that of
other ions. However, in AAEMs, the motion of OH can be
retarded by the polymer framework where the effective mobility
of OH is oen much lower than that in dilute solutions. This is
a common drawback of polymer electrolytes including Naon®
(where the effective mobility of H+ is only about 20% of that in
dilute acids).
The conduction of ions, such as H+ or OH, relies on the
presence of water so the structure of hydrophilic domains in a
polymer electrolyte is the predominant factor for ion
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conduction. It is believed that the outstanding ionic conductivity of Naon® is attributed to its phase segregation
morphological structure.99 Specically, the presence of both a
highly hydrophobic uorocarbon polymer backbone and exible side chains (that contain the ionic groups) drives the
formation of a hydrophilic/hydrophobic phase separation
structure, where ion-containing hydrophilic domains overlap
and form interconnected ionic channels. Although the nominal
IEC of Naon® is only ca. 0.92 meq. g1, the localised H+
concentration in the ionic channels is much greater, which
signicantly increases the efficiency of H+ hopping conduction.
Since the OH conduction operates via a similar mechanism
to H+ conduction,100 a phase segregated (self-assembled) structure is expected to improve ion conduction in AAEMs.98
However, the formation of phase segregated structures in AEMs
is more challenging as most AEMs are based on hydrocarbon
backbones with lower hydrophobicities compared to uorocarbon-based AEMs. The hydrophobicities are oen even lower
again as the cationic groups are commonly connected to the
hydrocarbon backbones via short links (oen –CH2–), e.g. the
quaternisation used to form QA polysulfone AAEMs markedly
changes the alkali stability and hydrophobicity of the polysulfone backbone (relatively hydrophobic when unmodied).
Elongating the length of the link between the polymer backbone
and the cationic functional groups should, in principle, assist in
the formation of phase segregation structures. However, this
requires an entire change in material synthesis methods as a
signicant number of AEMs reported in the literature are
prepared using a polymer modication protocol; for example, a
commercially available polymer (such as polysulfone) is functionalised with a reactive group (commonly –CH2Cl formed
using some form of chloromethylation reaction [oen using
highly carcinogenic reagents such as chloromethylether]) and
then further reacted (with reagents such as trimethylamine) to
yield the nal AEM containing polymer bound cations (Scheme
6a).101 This typical process is not easily adapted to yield
Naon®-like pendent cations (i.e. a QA attached to the polymer
backbone through a long side chain).
Phase segregated morphologies generally exist in block
(Scheme 6b/d/e/g) and gra copolymers (Scheme 6f),46a,102 which
results from the enthalpy associated with the demixing of
incompatible segments.103 Regarding the development of
copolymers, it is clear from the literature that the formation of
phase segregated morphologies is much more successful for
block copolymers compared to random copolymers (if comparable systems are compared).14a,49,104 For example, the phase
separated morphology of a polysulfone block copolymer (IEC ¼
1.9 meq. g1, high l ¼ 32 value [l values give the number of
water molecules per cationic head-group] has been reported to
give a very high hydroxide conductivity of 144 mS cm2 at 80 C
(over 3 times higher than an IEC ¼ 1.9 meq. g1 random
copolymer benchmark).104e Coughlin et al. have shown that
block copolymers can yield well dened lamella phase separation mophologies.23b Such high level organisation is, however,
not mandatory given that peruoro QA AEMs can also phase
separate (just like peruorosulfonic acid [PFSA] PEMs like
Naon®).16a QA-functionalised poly(hexyl methacrylate)-block-
3144 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
poly(styrene)-block-poly(hexyl methacrylate) systems have also
been shown to possess a highly developed phase separation
(using SAXS and TEM techniques) and this yielded relatively
high OH ion diffusion coefficients (comparable to PEM
benchmarks).105 However, due to the insufficient mechanical
strengths, such olen types can only serve as models to assess
the effects of molecular architecture on performances.
Beyer et al. reported strongly self-segregating covalently
crosslinked triblock copolymers with high conductivities (120
mS cm1 at 60 C for the fully hydrated sample with IEC ¼ 1.7
meq. g1 and l ¼ 72).106 Similarly, Bai et al. have reported
conductivities >100 mS cm1 at 60 C and >120 mS cm1 at 80
C for QA-PPO/polysulfone/QA-PPO triblock AAEM (IEC > 1.83
meq. g1, fully hydrated but with a much lower l ¼ 16).107
Coates et al. have also developed block copolymers but with
additional crosslinking (via the cationic groups) and this yielded AAEMs with equally exceptional OH conductivities (up to
110 mS cm1 at 50 C).21c Guiver et al. produced polysulfone
block copolymer AAEMs with higher OH conductivities at
lower l values, water uptakes, and dimensional swelling
compared to a non-block copolymer QA polysulfone benchmark
AAEM.108 Li et al. developed another class of block copolymer
where the QA group was separated from the polymer chain by a
triazole group (Scheme 6g).97 The triazole formation stemmed
from the use of Cu(I) catalysed “click chemistry”. This produced
AAEMs with excellent conductivities at room temperature when
fully hydrated: an IEC ¼ 1.8 meq. g1 AAEM gave a OH
conductivity of 62 mS cm1 (and an interestingly high CO32
conductivity of 31 mS cm1). However, the addition of triazole
links increased the water uptakes (cf. triazole-free examples).
Binder et al. have also developed “comb shaped” block
copolymers where the QA groups contained a long hydrocarbon
tails (Scheme 6d).109 AAEMs with OH conductivities up to 35
mS cm1 (room temperature, fully hydrated, IEC ¼ 1.9 meq.
g1) were reported. Even more interestingly, the water contents,
l, appeared to be independent to IEC (l ¼ 5.2–5.9 over the IEC
range 1.1–1.9 meq. g1); these were much lower than a benchmark block copolymer AAEM where the QA was a polymer
bound trimethylammonium (IEC ¼ 1.4 meq. g1, l ¼ 10.4, OH
conductivity ¼ 5 mS cm1). Hickner et al. also investigated
“comb shaped” AEIs for APEFCs where an increasing number of
long alkyl chains (C6, C10 and C16 in length) were attached to
the QA groups.50 Higher performances were obtained with the 1
C16 AEI (IEC ¼ 1.65–1.71 meq. g1, 21 mS cm1 at room
temperature in water) but this AEI had less in situ durability
compared to a different 1 C6 example (IEC ¼ 2.75–2.82 meq.
g1, 43 mS cm1). The AEIs with multiple long alkyl chains on
the QA groups exhibited lower conductivities and water uptakes
compared to AEIs of similar IECs that contain only a single long
QA alkyl chain. Hickner et al. also showed that introducing
crosslinking into comb shaped AEMs can enhance their
stability towards alkali.110
Recent studies by Xu et al. investigated gra copolymers
(Scheme 6f) for AAEMs, which displayed superior fuel cell
related properties.102c Dimethyl-PPO-based copolymers with
poly(vinylbenzyl trimethylammonim) gras were synthesized
via atom transfer radical polymerization (ATRP).111 AAEMs with
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Scheme 6 General strategies for the development of phase segregated AEMs (b–g) compared to a benchmark homopolymer system (a). The
rectangles represent a polymer block.
OH conductivities up to 100 mS cm1 at 80 C were produced
with high gra densities and optimised gra lengths (IEC ¼ 2.0
meq. g1). Knauss et al. have also produced a PPO block
copolymer AAEM but where the hydrophobic blocks contained
additional phenyl side-groups (not aliphatic hydrocarbon side
chains). A high OH conductivity of 84 mS cm1 was obtained
with an IEC ¼ 1.3 meq. g1 AAEM.14a Importantly, this
conductivity was produced with the AAEM in a RH ¼ 95%
environment (rather than the normally encountered fully
hydrated condition where the AAEM is fully immersed in water).
Recently, Zhuang et al. reported a new and simple method
for achieving highly efficient phase segregation in a polysulfone
AEM.112 Instead of elongating the cation-polymer links or adding hydrophobic chains to the QA groups, long hydrophobic
side chains were directly attached to polymer backbone at
positions that are separated from the cationic functional group
(Scheme 6c). This polysulfone phase segregated AEM was
designated aQAPS. This structure is not categorised as a block
copolymer, but rather a “polysurfactant” where the hydrophilic
cationic head-groups (e.g. QA) are linked through the polymer
backbone but the long hydrophobic tails are freely dispersed.
This concept was inspired by the structure of Gemini-type
surfactants, where enhanced ensemble effects are seen when
properly tying up two single surfactant molecules.113 The effect
of phase segregation of the aQAPS design was identied using
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TEM and SAXS data (Fig. 2). The TEM image of the QAPS
polysulfone copolymer benchmark, where there was no hydrocarbon side chain on the hydrophobic blocks (analogous to
Scheme 6b), was uniform (Fig. 2a), which indicates the lack of
clear phase segregation. However, the hydrophilic domains in
the aQAPS system (dark zones in Fig. 2b, dyed using I before
TEM measurement) were clustered and separated from the
hydrophobic polymer framework (light background in Fig. 2b).
This strong phase segregation resulted in a long-distance
structural ordering, as indicated by the SAXS pattern (Fig. 2c).
As a consequence of the phase segregation, the ionic
conductivity of aQAPS was 35 mS cm1 at 20 C and >100 mS
cm1 at 80 C (Fig. 3) in comparison to non-phase-segregated
QAPS (15 mS cm1 at 20 C and 35 mS cm1 at 80 C). These are
very high conductivities for such a low IEC AAEM (1 meq. g1). The
ionic conductivity of aQAPS(OH) at room temperature was ca.
57% of that of Naon® (very close to the mobility ratio between
OH and H+ in diluted solutions). This indicates that the ionic
channels in aQAPS are as efficient as that in Naon® (i.e. the
difference in ionic conductivity being just the mobility difference between OH and H+). However, at temperatures that are
more relevant to fuel cell operation (60–80 C), the IEC normalised ionic conductivity of aQAPS(OH) was as high as that of
Naon®(H+). This shows that the OH conduction can be as
fast as that of H+ at elevated temperatures, provided the ionic
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AAEMs in (chemical) fuel cells2
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H2/air(O2) alkaline polymer electrolyte fuel cells (APEFCs)
Fig. 2 TEM images of polysulfone-based AEMs: (a) QAPS [Scheme 6b]
and (b) aQAPS [Scheme 6c]. (c) The resulting SAXS patterns.112
channels in the AAEM are optimised. This signicant nding
demonstrates that OH conductivities in AAEMs are not
intrinsically inferior those of H+ in PEMs.
The need for AEIs in electrochemical systems
Before the discussions move onto application specic items, the
subject of the need for solubilised/dispersible AEIs needs to be
introduced. To maximise the catalyst utilisation (optimal triphase interface [gas diffusion + ionic conduction + electronic
conduction pathways available to a maximum amount of catalyst surface]) and introduce the required level of ionic conduction and hydrophobicity into the electrodes of low pH
electrochemical devices such as PEMFCs, Naon® dispersions
(e.g. D-521/D520) are commercially available, scientically well
known, and widely used as acidic ionomers.99,114 For AEM/AAEM
containing systems, the availability of commercial-grade AEIs is
more restricted and less optimised for application in electrochemical applications. The usage of Naon® CEM ionomers
with AAEMs in APEFCs is a non-ideal situation.115 Both
Tokuyama116 and Fumatech have developed AEIs.117 Other
researchers have developed their own concepts or solubilised
the materials used to make the AAEMs themselves (where possible).15c,28b,118 However, it is important to keep in mind that if
production of an AEI technology is to be scaled up (for commercialisation) then it is vital that the AEI is supplied most
desirably in an aqueous-based form (dispersion or solution).
This is for safety considerations: the presence of both organic
solvents and large quantities of nely divided (nano) catalysts in
the scaled up manufacture of MEAs present will present a
signicant hazard.119
AAEMs and AEIs are used in APEFC technology.2 In the literature this class of fuel cell is also called Alkaline Membrane Fuel
Cell (AMFC), Anion Exchange Membrane Fuel Cell (AEMFC), or
Solid Alkaline Fuel Cells (SAFC). In principle APEFCs are similar
to PEMFCs, with the main difference that the solid membrane is
an AAEM instead of a PEM. With an AAEM in an APEFC, the
OH is being transported from the cathode to the anode,
opposite to the H+ conduction direction in a PEMFC. The
schematic diagram in Fig. 4 illustrates the main differences
between the PEMFC and the APEFC. In the case of a PEMFC, the
H+ cations conduct through a solid PEM from the anode to the
cathode, while in the case of an APEFC the OH anions (or other
alkali anions – see later) are transported through a solid AAEM
from the cathode to the anode. The use of solid electrolytes also
prevents electrolyte seepage, which is a risk with traditional
alkaline fuel cells (AFCs) that use aqueous Na/KOH electrolytes.
The ORR121 and hydrogen oxidation reaction (HOR) for a
PEMFC (HOR ¼ eqn (6) and ORR ¼ eqn (7)) are compared to an
APEFC (HOR ¼ eqn (8) and ORR ¼ eqn (9)) below [recall, all E
values are given as reduction potentials even if a reaction is
written as an oxidation]:
2H2 / 4H+ + 4e E ¼ 0.00 V vs. SHE
(6)
O2 + 4H+ + 4e / 2H2O E ¼ 1.23 V vs. SHE
(7)
2H2 + 4OH / 4H2O + 4e E ¼ 0.828 V vs. SHE
(8)
O2 + 2H2O + 4e / 4OH E ¼ 0.401 V vs. SHE
(9)
2H2 + O2 / 2H2O Ecell ¼ 1.23 V (both acid and alkali)
(10)
Although the overall reaction (eqn (10)) is the same for both
types of fuel cells, the following differences in both technologies
are very important:
Fig. 4 Schematic comparison of a proton exchange membrane fuel
Fig. 3
IEC normalised conductivities of Nafion®, aQAPS, and QAPS.112
3146 | Energy Environ. Sci., 2014, 7, 3135–3191
cell (PEMFC, left) and an alkaline polymer electrolyte fuel cell (APEFC,
right) that are supplied with H2 and air.120
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(a) Water is generated at cathode side of PEMFCs but is
generated at the anode in APEFCs;
(b) While there is no need for water as a direct reactant in
PEMFCs, water is a reactant in APEFCs as it is consumed in the
cathode reaction.122
In principle, the advantages of APEFCs over PEMFCs are
related to the alkaline pH cell environment of the APEFCs:
(a) Enhanced ORR catalysis, allowing for the use of less
expensive, Pt-free catalysts such as those based on inorganic
oxides including perovskites, spinels and MnOx, as well as those
based on Fe, Co, Ag, and doped graphene (among others);123
(b) Extended range of (available) cell and stack materials
such as cheap, easily stamped metal (e.g. Ni and uncoated
stainless steel bipolar plates);
(c) A wider choice of fuels in addition to pure H2 (e.g.
hydrazine hydrate and “dirty” H2 including H2 containing traces
of ammonia – see later sections).
The most critical concerns for APEFC technology are the low
conductivities and the relatively poor stabilities of the AAEMs
that were developed in the rst years of the APEFC development.2,124 However, as discussed above, signicant advances
have been made in recent years that have promoted APEFC
development.
Electrocatalysts for H2-based APEFCs. The reader should
rst refer to the paper by Gasteiger et al. if they require detailed
discussion on the issues and considerations of benchmarking
fuel cell catalysts (including non-Pt types).125 With the latest
advances in conductive and alkali stable AAEM for fuel cells, the
need for the research into developing suitable catalysts has
increased in priority. While retaining the advantages of PEMFCs
(e.g. all solid state), APEFC technology opens the door for the
use of non-precious and cheaper catalysts,123,126 which yields the
potential for overcoming the high fuel cell cost barrier.
However, the eld of electrocatalysts for both the cathode and
anode in APEFCs is only now being explored in detail.127 With
the development and application of non-Pt catalysts, their
stabilities also need to be considered.128 Moreover, little has
been done with real APEFCs containing catalysts others than Pt.
Oxygen reduction reaction (ORR) catalysts.129 The ORR
overpotentials in APEFCs are similar to those in PEMFCs, (i.e.
the cathode overpotential loss remains an important factor
limiting the efficiency and performance of an APEFC).130
However, switching to an alkaline medium (as in APEFCs)
allows for the use of either a low level usage of Pt-group metal
(PGM) catalysts or a broad range of non-PGM catalysts with ORR
activities similar to that of Pt. Jiang et al. reported that the ORR
activity of a Pd coated Ag/C catalyst in alkaline medium was
three times higher than the corresponding activities on the Pt/C
(measured using ex situ rotating disk electrode tests).131 Piana
et al. reported that the specic current of Acta's Hypermec™
K18 (Pd-based) catalyst132 is about 3 higher than Pt/C and its
Tafel slope is also lower; the latter is also observed with other
non-Pt catalysts.133 He et al. reported that the kinetic current
density of a non-PGM catalyst based on CuFe–Nx/C was
comparable, or even higher, than a commercial Pt/C catalyst.134
The development of non-PGM ORR catalysts that are designed
specically for use in APEFCs now requires further research in
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Energy & Environmental Science
order to make this a real affordable technology. Carbon-free
catalysts should be considered, as carbons are active for the
peroxide generating (n ¼ 2e) ORR in alkali:
O2 + 2e + H2O / HO2 + OH
(11)
Alternatively, catalysts that are active in reduction of
peroxide at low overpotentials (bi-functional catalyst) are also
deemed advantageous for alkaline systems.123
Hydrogen oxidation reaction (HOR) catalysts. Whereas
research on ORR catalysts in alkali has now begun, studies on
the HOR catalysts for APEFCs constitute a relatively unexplored
eld. The kinetics of the HOR on Pt catalysts in PEMFCs (at low
pH) is so fast that the cell voltage losses at the anode are usually
considered negligible.135 This is not the case in APEFCs and the
anode performance is oen much poorer than the cathode
performance (with Pt catalysts in each).127b,130,136
In one of the very few studies investigating HOR activities of
platinum in both acidic and alkaline media, Sheng et al. found
that in alkaline electrolyte the HOR kinetics are several orders of
magnitude slower than in acid electrolyte.130 More recently, this
nding has been conrmed and quantied by Rheinländer et al.
who reported that ultra-low loadings Pt in aqueous KOH (1 mol
dm3) might exhibit prohibitively large losses of 140 mV at 40
mA cm2.137 Moreover, when looking for non-Pt HOR catalysts,
it was found that Pd-based catalysts exhibit 5 to 10 lower
activity than Pt in alkaline medium.137 However, a recently
reported PdIr/C catalyst showed a HOR activity comparable to
Pt/C.138
Rare studies investigating non-PGM catalysts for HOR in
H2/O2 APEFCs have been carried out by Zhuang et al.127a,c The
authors reported that by decorating Ni nanoparticles with Cr,
they succeeded to tune the electronic surface of Ni, making it
possible to operate in the anodes of APEFCs. They reported a
single preliminary test in real APEFCs, showing a maximum
peak power of 50 mW cm2 at a cell temperature of 60 C (Nibased anode and Ag-based cathode). Although the power
densities were still low, these are the rst published reports on
APEFCs that used non-Pt catalysts for both the HOR (anode) and
the ORR (cathode) in a single cell, hence they demonstrate the
potential of the APEFCs. A more recent ex situ experimental and
theoretical study by Yan et al. indicates that CoNiMo catalysts
hold promise for use as a HOR catalyst in APEFCs with the
potential to outperform Pt-catalysts (when at high loadings).139
All of these fundamental HOR studies strongly emphasise the
need for alternative, inexpensive HOR-catalysts for the
successful development of the technology. This is a major
research priority.
In all cases reported, however, the stability (and durability) of
the non-Pt HOR catalyst has been a major limiting factor. All
authors of published works suggest morphological changes in
the process of catalyst operation as a major source of instability
of the interface. The challenge in practical non-Pt HOR design is
that no catalyst has been shown to be active at comparable rates
in both the HOR and hydrogen evolution reaction (HER). The
search for a true breakthrough continues!
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The effect of AEI-bound cationic groups on the electrode
reactions. To recap, more research is required to increase the
fundamental understanding of electrocatalysts in alkaline
medium, especially as little research has been conducted into
effective catalysts that are specically developed for use in
APEFCs. For ex situ experiments, it is important that experiments are conducted with all-solid-state cells (i.e. not using
aqueous electrolytes containing spectator ions such as K+ and
excess OH). This will yield more fuel cell relevant electrochemical activities and oen reveals electrochemical features
that were obscured in experiments in aqueous electrolytes.140
As the AEIs will be in intimate contact with the catalysts,
another consideration is the inuence of the cationic functional
groups on the electrochemical activities of the catalysts; this will
vary for each catalyst/AEI-cationic-group combination. Recent
ex situ electrochemical studies have investigated the effect of
fully dissolved cationic small molecules (not bound to polymers) on bulk polycrystalline141 and Pt/C ORR catalysts [1 mmol
dm3 cationic molecules dissolved in aqueous KOH (1 mol
dm3)].142 Even though these studies are not directly comparable to the in situ situation in APEFCs (i.e. Naon® [cationexchange] ionomer dispersion were used in the formulation of
the catalyst inks [rather than using AEIs as a binder] and
cationic molecules were fully dissolved in aqueous KOH electrolytes with excess spectator ions), the studies have provided
some useful indicators of issues that need to be considered:
(a) Unlike with acid electrolytes,143 Cl anions (the anion
present in all of the experiments) did not have a major effect on
the ORR on Pt (1 mmol dm3 Cl concentrations tested);
(b) Benzene-ring-free QA cations (e.g. tetramethylammonium) have a low impact on the ORR on Pt, whereas benzene-ringanalogues (e.g. benzyltrimethylammonium) lead to impeded
ORR performances;
(c) Imidazolium cations (e.g. benzyl-3-methylimidazolium)
lead to severe reductions in the ORR performance of Pt; these
cations also change the mechanism so the level of undesirable
(peroxide generating) n ¼ 2e ORR increases (eqn (11));
(d) Pt catalysts oxidise the organic cations at high potentials
and the degradation products may also have an impact of the
ORR performances (degradation of organic components at the
anode may also have an effect on the HOR). This also suggests
that research needs to be conducted into the electrochemical
stability of the cationic head-groups bounds to the AEIs that are
in contact with the catalysts (especially at higher cathode potentials). In this respect, Pt (that tends to catalyse a broad range of
things very efficiently) may well be the worst choice of catalyst;
(e) As expected,144 polycrystalline (bulk) Pt gave higher
specic activities (electrochemically active surface area normalised current densities) and exchange current densities than
Pt/C nanocatalysts (comparisons with each cation additive).
Similarly, a study by Shao et al. investigated the effect of 1methylimidazole and triethylamine (but not charged imidazolium and quaternary ammonium species) on the ORR and HOR
on Pt/C in aqueous KOH.145 Similarly, Konopka et al. looked at
the effect on the ORR of polycrystalline Pt of 1,1,3,3-tetramethylguanidinium cations.146
3148 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
Other studies have looked at the effect of AEIs themselves on
the performances of various catalysts towards various reactions
at high pH (where Naon® ionomer is not present). These
experiments are closer to the conditions in APEFCs. These
studies involved AEIs such a commercial QA types (Tokuyama
AS-4)145–147 in-house synthesised polysulfone-imidazolium
types,145 and a phosphonium type.123d A more recent study
investigated the effect of polymer backbone of QA-AEIs on the
performance of an iron-cyanamide-derived catalyst.148 The poly(phenylene) and Naon®-based AEIs led to higher performances compared to the polysulfone AEI. For another example,
the use of AS-4 and the imidazolium AEI both reduced the ORR
mass activity on Pt/C compared to the use of Naon® ionomer.145 However, the HOR mass activities were increased with
the use of AEIs compared to Naon® (with the imidazolium AEI
yielding the best HOR performance). Lemke et al. looked at the
use of AS-4 as the AEI with Ag-nanowire ORR catalysts.147 The
presence and loading of the AEI was observed to have an effect
on both the catalyst activity and the number of e per O2
reduction (n ¼ 2 [eqn (11)] vs. n ¼ 4 [eqn (9)]). Yan et al.
concluded that phosphonium cationic groups poison Ag ORR
catalysts much less when polymer bound (as part of the AEI)
compared to when part of dissolved small molecules.123d
Note: these prior studies did not compare AEIs containing
different head-groups but the same polymer backbone (and
IEC). The next stage of this series of investigations should
consider the ORR and HOR kinetics on fuel cell catalysts when
bound using AEIs containing different cationic head-groups
(QA vs. imidazolium etc.) in Naon®-ionomer-free systems (and
without addition of fully solubilised cationic molecules). An AEI
concept is now available that would facilitate such a study that
uses a selection of bulk producible AEIs containing different
cationic head-groups (but with the same IEC and polymer
backbone chemistry).149
The issue of CO2 in the air supplies (“carbonation”). One of
the desires of the AAEM community is to operate under ambient
conditions (i.e. with air supplies without prior CO2 removal).
Such operation is problematic at the APEFC cathode where ORR
occurs (eqn (9)) as OH is extremely reactive with CO2, rst
forming bicarbonate (eqn (1)) and then carbonate (eqn (2)).
Historically, CO32 anions have been thought of as a poison in
traditional AFCs that use aqueous KOH as the electrolyte since
K2CO3 has low solubility in water at room temperature (risk the
formation of precipitates in various parts of the AFC including
the electrodes).150 Even with the introduction of APEFCs, where
the CO32 and HCO3 species cannot precipitate (the positive
charge is part of the already solid electrolyte and there are no
mobile [e.g. metal] cations present), the reactions in eqn (1)/(2)/
(9) can still be problematic.
The trace CO32/HCO3 anions, generated at the cathode
from the reaction of the CO2 in the air supply and the OH
anions present in the electrolytes, diffuse away and accumulate
at the anode side of the MEA. This sets-ups an undesirable pH
gradient where the anode side of the MEA has a lower effective
pH (higher concentration of CO32/HCO3 species) than the
cathode side (retains a higher OH content than the anode
side);151 thermodynamically (cell voltage wise) it is better to have
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a lower pH at the cathode and a higher pH at the anode.
However, experimental and modelling studies43,152 show that
the CO32/HCO3 contents in the AAEMs and AEI can be purged
from the anode (“self-purging” and CO2 release) with the rapid
and continuous generation of the OH anions at the cathode at
high current densities. This self-purging can actually be exploited
if AEMs with suspected low stabilities to high concentrations of
OH [as in the commonly encountered aqueous KOH (0.5–1
mol dm3) ion-exchange solutions] are to be tested in APEFCs:
the AAEMs and AEIs can be initially converted to the CO32
forms, installed in the fuel cell, and then activated at high
currents (in situ conversion of the AAEM and AEI into the OH
forms).152
In fact, the situation may be even more complex: while
APEFC performances drop when the cathode supply is switched
from O2 to air, there is evidence that APEFC performances can
actually increase when CO2 is deliberately added to an O2
cathode supply (at higher CO2 concentrations than those found
in air).153 This intriguing effect certainly warrants more detailed
study. There is also a feeling in the APEFC community along
with some anecdotal evidence that being able to raise the APEFC
operating temperatures to > 80 C may well increase the tolerance
of these systems to CO2 in the air supplies (reduce the performance gap between APEFC operation with air compared to O2).
Again, this needs to be rigorously investigated, especially when
development of AAEMs/AEIs that are stable in the OH forms at
temperatures of >80 C for long periods of time has been
achieved.
H2-based APEFC performances. Recent developments in
highly conductive AAEMs have contributed to the growing
interest in APEFC technology. The last decade or so has seen the
rst reports of APEFC performances. A number of these reports
give performances high enough to show that the potential of
APEFCs needs to be seriously considered for practical application. Table 2 summarises key APEFC results reported in the
literature (with H2/O2 and H2/air systems). As can be seen,
maximum power densities of up to 823 mW cm2 have been
reported with O2 supplied cathodes and 500 mW cm2 for air
supplied cathodes. Open circuit voltages (OCV) are routinely
>1.0 V and OCVs as high as 1.1 V have been reported.
One of the best indicators of the potential of this technology
comes from the industrial sector. Aer all, there are commercial (fuel cell relevant) AAEMs available including those by
Tokuyama, Solvay and Fumatech.154 Tokuyama showed a
maximum peak power of 450 mW cm2 and 340 mW cm2 for
H2/O2 and H2/air (CO2 free) respectively at 50 C.43b Even
though those results were obtained with Pt/C catalysts (0.5 mgPt
cm2), they show a power high enough for practical applications (such as backup power for stationary applications
including in the telecoms industry). At same time, Kim reported interesting results with his polyphenylene based
membranes.155 With 3 mg cm2 Pt black catalyst in both the
anode and cathode, a maximum power density of 577 mW
cm2 (at 1 A cm2) and 450 mW cm2 (at z0.8 A cm2) at 80 C
with H2/O2 and H2/air conditions respectively. This power
density is very similar to that achieved and reported by Yanagi
and Fukuta.43b
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Energy & Environmental Science
Although these Pt-based APEFCs already exhibit performances that are good enough for practical fuel cell application,
the need for these performances (or better) with alternative and
inexpensive catalysts is paramount. One of the very few results
presented on H2-based APEFCs with non-Pt catalysts was
recently obtained at CellEra (an Israeli company developing
AMFCs).127b Using dry H2 and CO2 ltered ambient air, Dekel
reported a peak power density of 700 mW cm2 (at 1.5 A cm2)
with 3 barg/1 barg H2/air at 80 C with an APEFC containing a Pt
anode catalyst, and a peak power density of 500 mW cm2 (at
1.6 A cm2) with 3 barg/1 barg H2/air at 80 C with an entirely
non-Pt (condential catalysts) APEFC.127b This data also
demonstrates that AAEMs can withstand considerable pressure
differentials between the anode and cathode (CellEra have data
that shows this to be true with 3 bar differentials). Tokuyama
also report that their A201 AEM has a burst strength of 0.4
MPa.159
Performances of APEFCs can be increased with the use of
active water management,160 i.e. control of anode RHs with
different anode RHs used at different current densities (lower
anode RH at high current densities to prevent anode ooding –
recall the anode is where the H2O is electro-generated). Water
management and APEFC performances should increase with
the use of thinner AAEMs.161 However, our experience to date is
that AAEMs that are thinner than ca. 30 mm (e.g. Tokuyama A901
is ca. 10 mm in thickness)116b does not always lead to the
expected increases in performance (even with well optimised
APEFC systems). This mystery needs to be investigated further.
A major in situ problem with the use of AAEMs and AEIs is when
they are exposed to low RH environments in APEFCs: the
conductivities signicantly drop even with only small drops in
RH (i.e. AAEMs and AEIs are much more sensitive to drops in
humidity than PEMs and H+-conducting ionomers).52 The alkali
stabilities of the AAEM and AEIs also decrease dramatically with
lower hydration levels (OH is stronger nucleophile when not
fully hydrated). AEI degradation occurs mostly in the cathode
catalyst layer as this is where most dehydration occurs; it
appears to be hard to keep the cathode AEI hydrated even when
more than enough H2O is transported back from anode and the
cathode gas supply is fully hydrated.
In summary, extremely rapid and promising achievements
in H2 APEFC technology have been shown. Based on the above
described (recent) results that have been obtained with rst
prototypes, it seems that APEFC technology is not just a future
promise, but a present reality. APEFCs development has been
rapid in the past ve or so years where experimental work is now
being followed up with detailed theoretical and modelling
studies.43a,161,162 This technology promises to solve the cost
barriers (of PEMFCs), which is one of the main “pain-points” of
fuel cell technology. While H2/air APEFC cells and stacks have
achieved practical performances (for a few applications such as
back-up power), there are still development challenges, which
must be addressed to enable the large-scale introduction of
APEFCs. These mainly include:
(a) AAEMs and AEIs with improved stabilities and conductivities at higher temperatures and especially when exposed to
lower RH environments;
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Table 2
Perspective
Select literature data that present peak power densities measured with laboratory-scale H2-based APEFCs
Peak power
density achieved/
mW cmgeo2
Current density/
mA cmgeo2
(voltage/V) at
peak power
Gas supplies
anode/cathode
(back pressure)
77
70
180
z90
z40
138
196
28
30
230
50
823
506
158 (0.49)
170 (0.41)
400 (0.45)
z170 (0.53)
z60 (0.67)
z280 (0.49)
z380 (0.52)
60 (0.47)
z65 (0.46)
600 (0.38)
100 (0.50)
z1800 (0.46)
z1000 (0.51)
H2/air
H2/O2
H2/O2 (1 barg)
H2/O2 (2 barg)
H2/air (2 barg)
H2/O2 (2.5 barg)
H2/O2 (2.5 barg)
H2/air
H2/air
H2/O2
H2/O2 (1.3 barg)
H2/O2
H2/air (1 barg)
Catalyst type and
loading/mg cmgeo2
AAEM
Cell
temperature/ C
Anode
”Peruorinated piperazinium”
QA polysulfone
QA polystyrene copolymer
QA polyphenylene
70
70
70
80
Pt
Pt/C
Pt/C
Pt
Stabilised phosphonium
polysulfone
Crosslinked QA polysulfone
Crosslinked QA polysulfone
QA radiation-graed ETFE
QA polysulfone
QA radiation-graed low
density polyethylene
50
80
60
60
50
60
60
60
Pt
(b) Pt-free highly efficient catalysts, especially for the HOR.
Advances related the above two issues will assure rapid
entrance of APEFCs into existing market opportunities.
AAEM fuel cells fuelled with C-based fuel alcohols2d,l,o,163
There are several drivers for the use of alcohol fuels in AAEMbased fuel cells (primarily for portable power applications):
(a) Alcohols are liquid fuels with high volumetric energy
densities (Table 3), even when compared to cryogenic H2(l);
(b) The maximum theoretical efficiencies are widely stated to
be high when based on the higher heating value (>97% for
alcohols compared to 83% for H2 and 91% for NaBH4).
However, please consider that these numbers can be misleading
and not practical as only free energy in and free energy out (i.e.
exergy) matters;
(c) Because alcohol oxidation is generally more facile in high
pH environments164 and can be structure insensitive,165 there is
a large repertoire of potentially cheaper and more abundant
anode catalysts that can be used.
(d) A larger range of ORR catalysts increases the odds of
nding alcohol tolerant options (including MxOy types);123f,132,166
(e) The conductive ions (OH) move through the AAEM in a
direction (cathode / anode) that is contrary to alcohol crossover (anode / cathode). This may mitigate against alcohol (and
alcohol electro-oxidation product) crossover, especially at
higher currents. Alcohol oxidation at the cathode can lower the
cathode potential and/or poison the cathode catalyst. This is
different to the situation in PEM-based Direct Alcohol Fuel Cells
(DAFC) where the H+ ions move from anode / cathode thereby
enhancing alcohol crossover (due to electro-osmotic drag). This
is serious when materials such as Naon are used (with high
methanol permeabilities of >106 cm2 s1 and methanol
uptakes).167 Well designed and engineered systems, where the
alcohol concentration in the anode catalyst layers is low (due to
efficient and rapid oxidation), suffer less from alcohol crossover
effects;
3150 | Energy Environ. Sci., 2014, 7, 3135–3191
Pt/C
Pt/C
Pt/C
Ni–Cr
Pt/C
Pt/C
Cathode
References
0.5
0.4
0.4
2
Pt
Pt/C
Pt/C
Pt
0.5
0.4
0.5
2
156
2n
157
15c
0.2
0.5
0.5
0.5
0.5
5
0.4
0.4
Pt
0.2
0.5
0.5
2
0.5
1
0.4
0.4
39f
Pt/C
Ag/C
Pt/C
Ag
Pt/C
Pt/C
57f
57e
158
127c
25e
(f) In AAEM-DAFCs, H2O is consumed in the cathode catalyst
layers (recall eqn (9)) and electro-generated at the anode where
there is already a liquid reactant supply present (the reverse
situation to PEM-DAFCs). This change in water balance may be
benecial in reducing ooding at the cathode: there is electroosmotic drag of a large number of H2O molecules (>19) per H+
when aqueous alcohol solutions are supplied to the anode in
PEM-DAFCs (a signicant problem).168 However, modelling
suggests that insufficient H2O transports through the AAEM (to
the cathode) to sustain high currents in AAEM-DMFCs (Direct
Methanol Fuel Cell).169 Related to this, Smart Fuel Cells (Germany) has a patent for a “fuel cell combination”, where the
cathode of a PEM-DMFC is located next to the cathode of an
AAEM-DMFC. This is to efficiently utilise, in the AAEM-DMFC
cell, the large quantities of H2O transported through the
membrane (anode / cathode) of a PEM-DMFC cell.170
(g) A small amount of alcohol penetrating into the
membranes may protect the membranes (particularly hydrocarbon types) against peroxide attack.171 The presence of alcohols may also assist with the “cold start-up” of DAFCs (i.e.
starting up the fuel cell from sub-zero temperatures).172
Methanol, ethanol, ethylene glycol, and glycerol are the more
common alcohols to have been tested in AAEM-based fuel cells;
however, higher alcohols such as the propanols have also been
considered for electro-oxidation in alkali.173 The oxidation
reactions (in alkali) for methanol, ethanol, ethylene glycol, and
glycerol are given in eqn (12)–(16):
CH3OH + 6OH / CO2 + 5H2O + 6e
E ¼ 0.81 V vs. SHE
(12)
CH3OH + 8OH / CO32 + 6H2O + 6e
(13)
CH3CH2OH + 16OH / 2CO32 + 11H2O + 12e
E ¼ 0.75 V vs. SHE
(14)
HOCH2CH2OH + 14OH / 2CO32 + 10H2O + 10e
E ¼ 0.82 V vs. SHE
(15)
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Table 3
Energy & Environmental Science
Select properties of commonly encountered fuel options used in fuel cells
Fuel option
Specic energy
density/kW h kg1
Density at 20 C/g cm3
Volumetric energy
density/kW h dm3
Ecella/V
nb
H2 (l)
Methanol CH3OH (l)
Ethanol CH3CH2OH (l)
Propan-1-ol CH3CH2CH2OH (l)
Propan-2-ol CH3CH(OH)CH3 (l)
Ethylene glycol HOCH2CH2OH (l)
Glycerol HOCH2CH(OH)CH2OH (l)
Formate HCOO (s)
Hydrazine N2H4 (l)
Ammonia (l)
Ammonia borane H3N:BH3 (s)
Sodium borohydride NaBH4 (s)
Typical gasoline (l)
33
6.1
8.0
9.1
9.0
5.2
5.0
0.9
5.4
3.3
8.4
9.3
12.8
Gas
0.79
0.79
0.81
0.79
1.11
1.26
Solid
1.00
Gas
Solid
Solid (1.07e)
0.71–0.77
2.37 (0.53c)
4.8
6.3
7.4
7.1
5.8
6.4
—
5.4
3–5d
—
(10e)
9.5
1.23
1.21
1.15
1.13
1.12
1.22
1.09
1.45
1.62
1.17
1.62
1.64
—
2e
6e
12e
18e
18e
10e
14e
2e
4e
3e
6e
8e
—
a
Maximum thermodynamic cell potential at 25 C on oxidation in a cell with O2 as the oxidant. b The number of e per fuel molecule obtained on
full oxidation. c Gaseous H2 at 200 bar. d Depending on the conditions. e For a solution of 500 g NaBH4 in 1 dm3 solution at 25 C. (l) ¼ liquid and
(s) ¼ solid.
HOCH2CH(OH)CH2OH + 20OH / 3CO32 + 14H2O + 14e
E ¼ 0.69 V vs. SHE
(16)
Alcohol oxidation (assuming full electro-oxidation) produces
CO2 leading to signicant concentrations of CO32/HCO3 in
the anodes of the AAEM-based DAFCs. The resulting pH
gradient (high pH cathode / low pH anode) will produce
thermodynamically derived voltage losses.151 It has been calculated that a pH difference of 6.1 would exist across the AAEM at
20 C corresponding to a thermodynamic voltage loss of ca. 360
mV, which drops to a pH difference of 4.1 (voltage loss of ca. 290
mV) at 80 C. Such voltage losses can be offset with the
improved kinetics at temperatures $80 C (especially if alcohol
crossover is suppressed).
As will be evident from the below discussions, acceptable
power performances are only obtained when large amounts of
Na/KOH are added to the aqueous alcohol fuel supplies.
Assuming full oxidation of the alcohols, the presence of such
additional quantities of additional OH will lead to CO32 being
the predominant product (rather than CO2):58 hence eqn (13)–
(16) are written as such. As the point of using AAEMs/AEIs in
fuel cells was to eliminate the use of aqueous Na/KOH, it must
be questioned if the use of AAEMs is needed when the addition
of Na/KOH to the fuel supplies cannot be avoided.174 Aer all,
methanol has been used as a fuel in AFCs in the past. The review
by Koscher and Kordesch details historic work involving alkaline methanol–air systems with liquid caustic electrolytes.175
AAEM-based DMFCs. Methanol is the simplest alcohol (a C1
alcohol with no C–C bonds to break) that has been evaluated in
AAEM-based fuel cells.176 This includes being used as a fuel in a
microfabricated passive AAEM-containing fuel cell.177 Despite
this chemical simplicity, the power densities obtained with C2+
alcohols such as ethanol and glycerol are similar to those
obtained with methanol (see later). The 6e methanol oxidation
reaction (MOR) mechanism is still more complex compared to
the 2e HOR and the rate determining step is thought to be the
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oxidation of the –CHO intermediate species.2l Bi-functional
catalysts such as PtRu, that are required as anode catalysts in
PEM-DMFCs for the removal of strongly bound Pt–CO intermediates (facilitated by the presence of adjacent Ru–OH sites),
may not be required (at least in such high amounts) for AAEMDAFC anodes due to the high concentration of OH anions
present.178 High concentrations of CO32/HCO3 species at the
anode catalysts in AAEM-DMFCs may or may not interfere with
the MOR (this needs to be studied in more detail).179,180 Along
similar lines to the studies that are looking into the effect of AEI
ionomers (or model small molecules) on the ORR on various
catalysts, it has been reported that benzyltrimethylammonium
has a stronger negative affect towards the MOR on Pt/C in
aqueous KOH (0.1 mol dm3 containing 0.05 mol dm3 methanol) compared to tetramethylammonium over the additive
concentration range 1–120 mmol dm3.181 Again, the effect of
the AEI being used on the MOR for the catalyst being considered
for application needs to be evaluated.
Alongside the many studies with Pt-based catalysts,182
183,184
Pd
and Au185 catalysts have also been considered for MOR
in alkali. However, due to cost, the big push has been towards
the development of non-PGM catalysts. Ni-based catalysts are a
commonly encountered option.186 Spinner et al. have studied
NiO-based catalysts for MOR in alkali. They found that activities
were higher in aqueous CO32 electrolyte compared to aqueous
OH electrolytes.180 Minteer et al. have reported that the inclusion of magnetic composites into Ni-based electrocatalysts can
boost MOR in alkali.187 Perovskite-based MOR catalysts have
also been considered.188
As well as the conductive species opposing methanol crossover in AAEM-DMFCs, there have been many studies into
nding AAEMs with low methanol (and other alcohol) permeabilities.10f,65,189 High methanol diffusion through the anode
AEI is, however, desirable.190 Many of these studies mention an
ex situ “selectivity” or “DMFC performance” parameter, which is
the ratio of 2 intrinsic properties:10f,65,102b,191 ionic conductivity to
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methanol permeability (¼ s/PMeOH). An ideal AAEM will have a
high ionic conductivity and low methanol permeability yielding
a high performance parameter. However, caution is required
with this parameter as an AAEM with a high performance
parameter value but with a very low ionic conductivity will not
be suitable for application (i.e. a low conductivity material with
an extremely low methanol permeability). Obviously there will
be analogous ex situ performance parameters for the other fuel
options detailed below.
Without the addition of metal OH (MOH) salts to the
methanol anode supply, the performances of AAEM-DMFCs are
generally poor with typical peak power densities of <20 mW
cm2 (even with reactant pressurisation).115,174,192 The chances of
6/8 OH anions diffusing/migrating through the hydrated
components of the anode AEI at the same time (to a localised
site on the catalyst surface) to allow rapid oxidation of a single
methanol molecule is deemed very low in the absence of an
additional source of OH ions. This is different to PEM-DMFCs
where the 6 H+ (generated on oxidation of a methanol
molecule) have to transport away from the anode catalyst
surface sites. Despite this, Benziger et al. report a reasonable
methanol (2 mol dm3 and OH-free)/O2 AAEM-DMFC performance with a peak power density of 31 mW cm2 (OCV ¼ 0.84 V)
at room temperature using an imidazolium-PEEK AAEM (95 mm
thickness and IEC ¼ 2.0 meq. g1) and AEI and Pt catalysts with
loadings of 0.5 mgPt cm2 (the catalyst coated membrane [CCM]
method was used).10a Analytical modelling of AAEM-DMFCs
suggests that MOH-free performances can be improved when
the anode side faces upwards due to more facile removal of the
CO2 bubbles (higher temperatures also facilitate CO2 bubble
removal).193
The performances are generally higher when MOH is added
to the aqueous methanol fuel supplies. The main contributor to
this improved performance is an improved anode potential
(>300 mV reduction in anode overpotential is possible). Katzfuß
et al. obtained a peak power density of 132 mW cm2 (OCV ca.
0.9 V) in AAEM-DMFCs at 80 C containing both an in-house
synthesised DABCO-crosslinked PPO AAEM (10 mm thick, IEC ¼
1 meq. g1, 80% degree of crosslinking) or a Tokuyama A201
AAEM (28 mm thick);14b the fuel cells contained an Acta 4010 (6%
Pd/CeO2/C) anode catalyst, and Acta 3020 (4% FeCo/C) cathode
catalyst and were supplied with aqueous methanol (4 mol
dm3) containing KOH (5 mol dm3) at the anode and dry O2 at
the cathode. The same group obtained a similar performance
(120 mW cm2) under the same test conditions using a 4
component DABCO-crosslinked PBI-polysulfone membrane
that was more stable to alkali than the prior DABCO-crosslinked
PPO AAEM.194 Prakash et al. reported a methanol/O2 AAEMDMFC containing Tokuyama's A201 AAEM and AS-4 AEI that
yielded a higher peak power density of 168 mW cm2 (OCV ¼
0.9 V) at 90 C when supplied with an aqueous methanol (1 mol
dm3) anode feed containing KOH (2 mol dm3).116a However,
these performances were obtained using PtRu (HiSpec 6000)
anode and Pt (HiSpec 1000) cathode catalysts (cell optimised
with a teonised cathode and non-teonized anode).
AAEM-based direct ethanol fuel cells (DEFC).2d,195 Unlike
with PEM-based systems, the performances of AAEM-DEFCs are
3152 | Energy Environ. Sci., 2014, 7, 3135–3191
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as high with ethanol as they are with methanol. The interest in
ethanol stems from its low toxicity (in moderation!), its higher
boiling point (cf. methanol), and its high volumetric energy
density. The use of lignocellulosic bioethanol may be a potentially “carbon-neutral” fuel option (i.e. next generation bioethanol that is not derived from food-based crops).196 However,
ethanol (C/O ratio ¼ 2) contains a C–C bond that needs to be
broken if full 12e oxidation to CO2/CO32 is to be achieved.
Partial ethanol oxidation (EOR) to [toxic] acetaldehyde (2e
oxidation) and acetic acid/acetate (4e oxidation) predominates
at temperatures of <100 C and this lowers the efficiencies of the
DEFC (less e from each ethanol molecule than the maximum
possible).197 This is a major challenge for DEFCs. However,
without full oxidation of alcohols (to CO2/CO32), the degradation of fuel cell performance is thought to be less of a problem
when using AAEMs;2o full oxidation would involve adsorbed CO,
which is an intermediate of full alcohol oxidation and a major
Pt catalyst poison. It is generally believed that there is a higher
chance of achieving full ethanol oxidation in alkaline systems
compared to acid systems, although the formation of CO32
instead of/as well as CO2 can make product analysis more
difficult with techniques such as DEMS.198
As such, there has been a lot of research into more active
EOR catalysts for use in alkali media.199 As can be expected, Ptbased catalysts have been considered.182c,200 Pd and Pd alloy
catalysts are of increasing prevalence in the literature as non-Pt
options where C–C bond breakage has been reported (under
conditions such as lower NaOH concentrations).164b,183,201 It is
believed that the sites of the Pd where there are adsorbed OH
(OHads) species are the catalytic active sites rather than the Pd
atoms themselves.202 However, the in situ durability testing (500
h) of Pd/C anode catalysts in AAEM-DEFCs show that the
performance losses observed are due to the growth in Pd
particle size (the performance of the Fe–Co cathode catalyst
used remained constant).203 Oxide supported Pd catalysts have
also been considered where Pd–NiO was reported to be a good
option for EOR.204,205 It is believed that the formation of OHads
on the metal oxide can transform carbonaceous species to CO2
at lower potentials (releasing Pd sites for further reaction).206
PdAu (90 : 10) catalyst yielded a higher AAEM-DEFC performance compared to PdAu catalysts with other ratios and Pdand Au-only benchmark catalysts. The presence of gold oxide/
hydroxyl species is thought to be important.207 Datta et al.
reported that PdAuNi catalyst produced improved AAEM-DEFC
performances compared to Pd-, PdNi, and Pd–Au benchmarks;208 this catalyst also produced higher yields of acetate and
CO32 (cf. acetaldehyde) compared to the other catalysts. Zhao
et al. have reported a PdIrNi high performance EOR catalyst for
AAEM-DEFCs.209 This catalyst performed much better compared
to Pd, PdNi and PdIr benchmarks. PdRu has also been studied
(see later).210 Li and He report that in situ reduction of Pd-layers
on Ni foam gave higher AAEM-DEFC performances compared to
conventional brushed Pd/Ni-foam anodes (164 mW cm2
compared to 81 mW cm2 at 60 C when tested in comparable
ethanol–O2 fuel cells containing a Tokuyama A201 AAEM).211
Au- and Ni-base catalysts (Pt- and Pd-free) have also been
considered as catalysts for EOR in alkali.129,212 As an example,
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RuNi catalysts have been reported to produce 8–9e per ethanol
molecule on average (mixture of H3COO and CO32 as
products).213
Sun et al. considered the use of an alkali-doped PBI
membrane (APM) for use in AAEM-DEFCs.214 An et al. compared
a CEM (Naon®-211), an AEM (Tokuyama A201), and an APM in
DEFCs.215 They concluded that AAEM had the best conductivity
and mechanical properties and the lowest Na+ permeability, the
CEM has the lowest ethanol permeability and the APM had the
best thermal stability but the poorest species permeability.
Overall the AAEM case was considered to have the best balance
of characteristics for application in DEFCs but the thermal
stability of the AAEMs need improvement.
In 2009, Bianchini et al. reported a AAEM-DEFC that gave a
peak power density of 170 mW cm2 at 80 C with the selective
production of acetate when fuelled with ethanol (10% mass) in
aqueous KOH (2 mol dm3) and with an active supply of O2 to
the cathode;216 the DEFC utilised an anode containing a
PdNiZn/C catalysts on Ni foam, an Acta Hypermec™ K-14
cathode, and a Tokuyama A201 AAEM. The performance dropped to a still respectable 58 mW cm2 in a passive (airbreathing) AAEM-DEFC at 20 C. More recently Chen et al. have
reported a peak power density of 176 mW cm2 (OCV > 0.8 V) in
a AAEM-DEFC at 80 C when fuelled with ethanol (3 mol dm3)
in aqueous KOH (3 mol dm3) and supplied with dry O2 at the
cathode;210 the fuel cell utilised a CCM containing a Tokuyama
A201 AAEM, a Pd3Ru/C anode catalyst on Ni foam (with a
Naon® ionomer binder), and a MnO2 nanotube cathode
catalyst (with Tokuyama AS-4 AEI). The enhancement of
performance over the use of a benchmark Pd/C anode catalyst
was attributed to the formation of RuOxHy at low potentials and
weak CO adsorption.
Zhao et al. reported an AAEM-DEFC peak power performance
of 185 mW cm2 at a lower 60 C temperature using a Acta “nonPt anode catalyst” supplied with ethanol (3 mol dm3) in
aqueous KOH (5 mol dm3) and a Pd3Au/CNT catalyst supplied
with O2 supply at the cathode;217 the DEFC contained a
Tokuyama A201 AAEM and Tokuyama A3 was used as the AEI.
The performance was higher than those obtained when using a
Pd/CNT or a Au/CNT cathode catalysts. The Pd3Au catalyst
where the Pd and Au were physically mixed gave better performances than the alloyed and core–shell analogues. This
performance was still lower than Zhao et al.'s alkaline-acid
DEFC concept where a peak power performance of 360 mW
cm2 at 60 C was obtained with a Naon-212 CEM, a PdNi/C–
Ni-foam anode, and a Pt/C (60% mass) cathode catalyst (Naon
ionomers used at both electrodes);218 the fuel supply was
ethanol (3 mol dm3) in aqueous NaOH (5 mol dm3) and
cathode supply was H2O2 (4 mol dm3) in aqueous sulfuric acid
(1 mol dm3).
AAEM-based directly ethylene glycol fuel cells (DEGFC).
Ethylene glycol (EG) is a low toxicity fuel option in AAEM-based
fuel cells where the molecules contain a single C–C bond as with
ethanol but where the C/O ratio ¼ 1 rather than 2.18c,219 Ex situ
studies show that the EG oxidation activities in OH and CO32
containing aqueous electrolytes (EG oxidation activity in OH >
in CO32) are superior to the activities seen with methanol in
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Energy & Environmental Science
OH electrolyte.220 EG oxidation activities are also generally
higher than seen with other polyhydric alcohols in alkali (EG >
glycerol > methanol > erythritol > xylitol).171 As with ethanol, the
full oxidation of EG molecules is rare and a variety of side
products are commonly observed: glycolate, glyoxalate, oxalate,
formate, glycol aldehyde and glyoxal.219c,221 However, full
oxidation of EG to CO2/CO32 appears to be easier than with
ethanol due to the lower C/O ratio, especially when applicationrelevant concentrations of the alcohol fuels are used.220,222 For
Pt, the partial oxidation pathway to oxalate is non-poisoning,
but the pathway to C1 species (CO and CO32) is poisoning due
to the adsorption of CO on the catalyst surface.220 Adding Bi and
Ni to the Pt anode catalyst decreases the level of C–C bond
breaking and directs the reaction to the oxylate pathway
(yielding higher activities).219c Pd/C catalysts that have been
stabilised with the addition of various oxides (e.g. Pd–Mn3O4/C)
have been shown to give good EG oxidation activities.204 A PdIn3
catalyst (synthesised using the sacricial support method) has
been reported with a very high EG oxidation mass activity.223 In
addition, cathode catalysts are available that are tolerant to EG
(e.g. perovskite types or Ag).171,219b
Zhao et al. have recently reported a AAEM-based DEGFC that
yielded a peak power density of 112 mW cm2 at 90 C when
supplied with O2 at the cathode and EG (1 mol dm3) dissolved
in aqueous KOH (7 mol dm3) at the anode.18c The fuel cell
contained an APM, PdNi/C/Ni-foam anode catalyst, an Acta
Hypermec™ non-PGM cathode catalyst. The peak power
density dropped to 90 mW cm2 when air was used instead of
O2 at the cathode.
AAEM-based direct glycerol fuel cells. Highly viscous and
non-toxic glycerol (C3 with a C/O ratio ¼ 1) has also been
studied as a fuel in alkaline AAEM-based fuel cells.184,224 This
interest partially stems from glycerol being an unwanted side
product from the production of biodiesel.225 Full oxidation of
glycerol has been achieved using enzyme cascades in an enzymatic fuel cell.226 However, in alkali, as with ethanol and EG,
glycerol tends to be only partially oxidised leading to the
production a wide variety of products: e.g. glyceraldehyde,
glycerate, glycerone (dihydroxyacetone), formate, glycolate,
hydroxypyruvate, oxalate, tartronate, and mesoxalate.227 Then
again, as glycerol contains 3 –OH groups, it is a recognised
feedstock for the production of a range of value-added chemicals and AAEM-based direct glycerol fuel cells may be useful for
cogeneration of energy and such chemicals.224e,228 An AAEMbased direct glycerol fuel cell has been recently shown to
selectively generate tartronate.224a
Glycerol can have higher ex situ activities on Au electrodes in
alkali compared to other alcohols (methanol, ethanol, and
ethylene glycol) and higher than on Pt.229 There is spectroscopic
evidence that full oxidation to CO2/CO32 may be achieved on
polycrystalline Au in alkali.230 A recent study into PdxBi catalysts
(synthesised using the sacricial support method) reports that
Pd4Bi displays the highest activity towards glycerol oxidation.231
This report shows that the catalyst is selective towards
production of aldehydes and ketones at low potentials,
hydroxypyruvate at medium potentials, and CO2 at high
potentials in the forward voltammetric sweeps. However, the
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catalyst is history dependent and carboxylates are selectively
produced on the reverse voltammetric sweep. Glycerol oxidation
mass activities are also high on PdIn catalysts.223
The power densities of glycerol fuelled fuel cells are generally
<200 mW cm2 and with low Faradic efficiencies (due to
incomplete oxidation to CO2/CO3). However, there are reports
of power densities of 265 mW cm2 being obtained.232 There are
even reports that glycerol can self-polymerise inside the fuel
cell. Li et al. obtained 184 mW cm2 at 80 C with an AAEMbased glycerol/O2 fuel cell containing a Tokuyama AAEM and
AEI, a Pt/C anode, and an Acta Hypermec™ non-PGM cathode;224c the anode was supplied with an aqueous solution containing crude glycerol (1 mol dm3) and KOH (6 mol dm3) at 30
psi back pressure. The use of crude glycerol (88% mass glycerol,
a by-product of soy-bean biodiesel production) did not produce
a drop in performance compared to much more expensive high
purity glycerol (99.8%). The Pt/C anode catalyst clearly had
stability in the presence of the impurities of the crude glycerol
(e.g. methanol, fatty acids [e.g. soaps], transesterication catalyst residues, and element such as K, Ca, Mg, Hg, P, S, As etc.).
Non-alcohol C-based fuels
Formate [HC(]O)O],233 glucose (C6H12O6),234 and urea
[H2NC(]O)NH2] and urine235 have also been studied as alternative C-based fuels in AAEM-containing fuel cells. Dimethylether has also been studied in fuel cells containing Naon®
CEMs but where performances were highest when the dimethylether was dissolved in aqueous alkaline anolytes (cf. acid
anolytes).236 The power densities achieved with glucose-fuelled
AAEM-based fuel cells are currently below 50 mW cm2. For
example, Zhao et al. achieved a peak power density of 38 mW
cm2 at 60 C when supplying a fuel cell containing a Tokuyama
A201 AAEM (PdNi anode and Acta Hypermec™ K14 cathode
catalysts) with aqueous glucose (0.5 mol dm3) containing
added KOH (7 mol dm3). Obviously, there is a wide range of
reaction pathways that are possible that will produce a variety of
electrochemical intermediates and products. 24e would be
needed for the full oxidation of glucose, which is highly unlikely
to occur. Glutonate (n ¼ 2e reaction) is the most common
product in alkali.234d The urea (urine) fuel cells containing
alkaline polymer electrolyte materials produced peak power
densities of <15 mW cm2:235 however, as with many of the fuels
mentioned above, this fuel cell concept is in a very early stage of
development.
AAEM-based direct formate fuel cells (ADFFC). The interest
in ADFFCs stems from the low (highly negative) anode potential
of the formate oxidation reaction (compared to methanol and
ethanol etc.):
HCOO + 3OH / CO32 + 2H2O + 2e
E ¼ 1.05 V vs. SHE
(17)
2HCOO + O2 + 2OH / 2CO32 + 2H2O Ecell ¼ 1.45 V (18)
Zhao et al. achieved a peak power density in an alkaline
direct formate fuel cell (ADFFC) at 80 C of 130 mW cm2 with
aqueous potassium formate (5 mol dm3) as the fuel and when
3154 | Energy Environ. Sci., 2014, 7, 3135–3191
using a Pd/C anode catalyst, commercial Acta Hypermec™ K14
cathode catalyst, QA polysulfone AAEM and AEI, and dry O2
supply to the cathode;233a this raised to >250 mW cm2 with the
addition of KOH (1 mol dm3) to the fuel supply. Haan et al.
achieved a peak power density of 267 mW cm2 at 60 C in a
ADFFC with a fuel supply consisting of HCOOK+ (1 mol dm3)
in aqueous KOH (2 mol dm3), a catalyst coated Tokuyama A201
AAEM (Pd black anode catalyst, Pt black cathode catalyst, and
Tokuyama AS-4 AEI), and a humidied O2 cathode supply;233b
the performance again decreased when the KOH was removed
from the fuel supply (157 mW cm2).
AAEM fuel cells supplied with N-based fuels237
Hydrazine and hydrazine hydrate.2p Hydrazine (H2N–NH2) is
a high volumetric energy density liquid fuel (at room temperature and atmospheric pressure) that contains 12.6% mass
hydrogen. N2H4 has also been used in the 1960s in traditional
AFCs including those that provided electric power in space
satellites.238 As early as 1972, the Government Industrial
Research Institute, Panasonic, and Daihatsu (all in Japan)
produced an experimental N2H4–air AFC vehicle.239 Hence,
N2H4 has been proposed for use in AAEM-based direct hydrazine fuel cells (DHFCs).29e,104e,118c,240 However, anhydrous N2H4 is
highly toxic (mutagenic/carcinogenic) and very unstable (a
rocket fuel). Technologies have been developed for practical
application where N2H4 is chemically xed to storage materials
such as polymers that contain carbonyl bonds [forming less
toxic hydrazone (>C]N–NH2) and hydrazide (–C(]O)–NH–
NH2) functional groups]. The N2H4 can released on addition of
solvents when required.241
Hydrazine hydrate (N2H4$H2O) is, however, considered
stable enough to be viable as a fuel for AAEM-DHFCs.242
N2H4$H2O is an industrially used chemical reagent (20 ktons
per year distributed in Japan) and it is less volatile than alcohols
(so air emissions will be potentially lower). It has a freezing
temperature of 50 C (so it can be easily used in cold climates)
and it has a of ame point of 74 C (at 1 atm). Hence, at aqueous
concentrations of <60%, N2H4$H2O is not ammable. The
carcinogenic risk of N2H4$H2O (class 2B by International
Agency for Research on Cancer) is equivalent to petroleum so
the careful handling of the fuel is no more than currently
accepted guidelines for petroleum.
The full (4e) electro-oxidation of N2H4/N2H4$H2O results in
the generation of harmless N2 and H2O products (eqn (19) – the
complete cell reaction is given by eqn (20) [along with the ORR
given in eqn (9)]), while H2 and NH3 are produced on partial
oxidation (eqn (21)–(24)).243 It has also been proposed that the
use of high pH conditions suppresses undesirable hydrolysis
reactions (eqn (25)).244 Unlike with the B-containing fuel vectors
(see below), no product (BO2) accumulation occurs at the
anode that requires spent fuel treatment, as all products are
gaseous. The potential of the N2H4/O2 cell reaction is larger
than the width of potential window of stability of water (on Pt)
and so there is a risk of the HER occurring (eqn (26)). Despite
this, OCV values in the range 0.8–1.0 V are typically observed.
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N2H4 + 4OH / N2 + 4H2O + 4e E ¼ 1.21 V vs. SHE (19)
N2H4 + O2 / 2H2O + N2 Ecell ¼ 1.61 V
(20)
N2H4 + OH / NH3 + ½N2 + H2O + e
(21)
N2H4 + OH / N2 + 3/2 H2 + H2O + e
(22)
N2H4 + 2OH / N2 + H2 + 2H2O + 2e
(23)
N2H4 + 2OH / N2 + ½H2 + 3H2O + 3e
(24)
N2H4 + H2O # N2H5+ + OH
(25)
4H2O + 4e / 2H2 + 4OH E ¼ 0.83 V vs. SHE
(26)
A variety of metal catalysts have been studied for use as
anode catalysts for N2H4/N2H4$H2O oxidation in DHFCs (Ni, Co,
Fe, Cu, Ag, Au, and Pt).245 Ni-based242,246 and Co-based29e,118c
catalysts appear most promising (Co catalysts have also been
examined in the cathodes). For example, Sakamoto et al. used a
combinatorial electrochemistry approach with 79 catalyst
candidates and report that Ni0.87Zn0.13, Ni0.9La0.1, Ni0.8Zn0.1La0.1, and Ni0.6Fe0.2Mn0.2 catalysts have N2H4$H2O oxidation
activities that are more active than a Ni/C reference catalyst.242a
Cu-based catalysts have also been proposed for use at the anode
of DHFCs.247 For example, a totally irreversible and diffusioncontrolled oxidation of N2H4 is reported on Cu-nanocubes on
graphene paper with N2 and H2O as the reaction products.248
The in situ formed copper hydroxide/oxide layers surface layers
on the Cu catalysts are thought to enhance the activity and
durability of the electrocatalyst. Selectivity to N2 and H2O as sole
products of hydrazine oxidation, as opposed to forming NH3
(eqn (21)), is of critical importance in practical DHFC development.242,246 The promise of such fuel cells as a “clean energy technology” will easily be compromised by a single digit ppm of NH3 in
the exhaust of a DHFC vehicle.
DHFC are liquid fuel fed fuel cells with a highly reactive fuel.
As a result, the fuel crossover is a natural concern of all designs.
The requirement for no hydrazine oxidation on the cathode is a
very strong one. As with other types of AAEM-based fuel cells,
non-PGM catalysts, such as Ag, can be used in the cathodes and
can have low N2H4 oxidation activities (mitigating against fuel
crossover effects).241
Peak power densities of over 600 mW cm2 have been achieved by Daihatsu Motor Co. Ltd. (Japan) in an AAEM-DHFC at
80 C containing a QA-polyolen AAEM with Ni anodes and Copolypyrrole/C cathodes (supplied with humidied O2) and a fuel
supply of an aqueous solution of N2H4 (5% vol) containing KOH
(1 mol dm3).243 As a comparison involving the use of H2O2 at
the cathode and a Naon®-117 PEM, a PEM-DHFC performance
of >1000 mW cm2 has been achieved at 80 C (PtNi/C anode
and Au/C cathode).249 Performances can be increased on
increasing NaOH concentration in the anode fuel supply up to
concentrations of 4 mol dm3 (reduced anode overpotentials).244 However, concentrations higher than these
reduce performances as viscosity increases and this causes
mass transport voltage losses at the cathode.
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Ammonia and ammonium carbonate. Ammonia (17.6%
mass hydrogen content) has also been studied as an alternative
fuel for AAEM-containing fuel cells.250 The complete electrochemical oxidation of NH3 (3e needed to oxidise each NH3)
and the related overall reaction (in alkali) are given by:
2NH3 + 6OH / N2 + 6H2O + 6e
E ¼ 0.77 V vs. SHE
(27)
2NH3 + 3/2 O2 / N2 + 3H2O Ecell ¼ 1.17 V
(28)
Power densities with NH3 have been low to date (<15 mW
cm2) with OCVs < 1.0 V; these values are lower compared to the
historic use of KOH as an aqueous electrolyte (ca. 50 mW
cm2).237 (NH4)2CO3 has also been considered for use in AAEMbased fuel cells but power densities were <1 mW cm2.251
A major problem is the very sluggish ammonia oxidation
reaction in alkali.252 Only PtIr, PtRu, and PtRh catalysts show a
reasonable ability to oxidise NH3 with the least serious affinity
towards the surface poison Nads. There can be recovery from
such Nads surface poisoning of Pt with H2 treatment of the
anode.250c The presence of Pt(100) surfaces appears to be
important.252a,f,g Reactive azides species may also be generated
as intermediates.252h Oxygenated (Oads and OHads) can form on
the catalyst surface with the presence of water and this (along
with Nads) also inhibits the catalytic performance.253 Interestingly, NH3 oxidation reaction is remarkably different on Pt
when studied in non-aqueous media (the formation of surface
oxygenated species is prevented) yielding N2 as the dominant
product and allowing the Pt to remain continuously active.253
This study also shows that Pd becomes highly active towards
NH3 oxidation in non-aqueous media (a low activity is seen with
Pd in aqueous KOH due to severe surface passivation).
APEFCs fuelled with H2 that is produced from NH3 reformation may have technological promise. AFCs can tolerate NH3
(several %) in the H2 fuel.254 However, PEMFCs are poisoned by
traces (1–2 ppm) of NH3 and performances decay (reversible but
only aer several days of operation with pure H2):255 the PEMs
themselves will slowly convert to the NH4+ forms lowering
conductivity and water contents.256 Hence, using H2 derived
from NH3 in PEMFCs would require prohibitively expensive
scrubbing of the reformed H2 supplies.
AAEM fuel cells fuelled with B-based fuels
Alkaline direct borohydride fuel cells.257 Sodium borohydride (NaBH4) is well known reducing agent in organic chemistry and a potential H2 storage material (contains 10.6% by
mass hydrogen). It is stable in concentrated aqueous alkali (t1/2
¼ 430 d at pH ¼ 14) but hydrolyses in acidic and neutral pH
environments yielding metaborate and H2:
BH4 + 2H2O / BO2 + 4H2
(29)
where BO2 is the empirical formula (Na/KBO2 typically contain
cyclic B3O63 anions). BH4 is being investigated as a fuel in
direct borohydride fuel cells (DBHFC). This includes mixed
reactant systems.258 A BH4/O2 fuel cell has a high theoretical
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energy density of 9.3 kW h kg1 as it can release a maximum of
8e per BH4. DBHFCs are of particular interest to the defence
industry for the portable power needs of the military and to
power vehicles (e.g. underwater vehicles) where the use of H2O2
liquid oxidants have also been proposed (a BH4/H2O2 fuel cell
has a theoretical energy density of 17 kW h kg1):259
4H2O2 + 8e + 8H+ / 8H2O
E ¼ 1.77 V vs. SHE (acid cathode)
(30)
Traditionally, the absence of AAEMs with adequate properties has meant that research efforts have traditionally focused
on using CEMs in the DBHFCs (in Na+ form when exposed to
aqueous solutions containing NaBH4 and NaOH); this includes
radiation-graed CEMs.260 This conguration leads to the buildup of NaOH at the cathode over extended operational times with
a concomitant and undesirable reduction of pH at the anode (in
the absence of an engineering solution that resupplies NaOH to
the anode from the cathode). However, CEM-DBHFCs can yield
reasonable fuel efficiencies due to minimised BH4 crossover
(CEMs are persmelective towards positive charged ions).
Replacement of the CEM with an AAEM allows a balance:
BH4 + 8OH / BO2 + 6H2O + 8e
E ¼ 1.24 V vs. SHE
BH4 + 2O2 / BO2 + 2H2O Ecell ¼ 1.64 V
(31)
(32)
where the OH anions are produced at the cathode (eqn (9)) and
consumed at the anode (eqn (31)). For both CEM- and AAEMbased systems, metaborate (not a major environmental
pollutant) accumulates in the fuel supply; it can, however, be
removed and converted back to BH4 but this is an energy
intensive process. Higher BH4 crossover rates are also now
observed with the use of AAEMs and this also limits the selection of cathode catalysts in AAEM-DBHFCs: Pt, Ag and Au
cathode catalysts cannot really be used in systems with appreciable BH4 crossover as they are active towards BH4 oxidation. MnOx-based ORR catalysts may be useful as they appear
inactive to BH4 oxidation.261 Operation at high current densities may mitigate against BH4 crossover to an extent due to a
higher ux of OH anions being transported from the cathode
/ anode and consumption of the BH4 in the anode catalyst
layers (lowering the localised concentration of BH4 concentration near the AAEM). The chemical stability of AAEMs is also
critical as they are exposed to high concentrations of BH4 and
OH: a typical anode supply contains $6 mol dm3 OH (to
ensure adequate BH4 stability). AAEM stability will be even
more critical if peroxide is used as an oxidant (Ecell ¼ 2.1 V with
HO2/OH cathode supply):
4HO2 + 4H2O + 8e / 12OH E ¼ 0.87 V vs. SHE
(33)
The hydrolysis reaction is a serious problem in DBHFCs and
is catalysed on many metals. This inevitably limits the selection
of an anode catalyst. The BH4 oxidation and hydrolysis side
reactions will happen in parallel to varying extents (as a
3156 | Energy Environ. Sci., 2014, 7, 3135–3191
function of temperature, concentration, catalyst and anode
potential etc.):
BH4 + H2O / BH3OH + H2
(34)
BH4 + 2OH / BH3OH + H2O + 2e
(35)
BH3OH + H2O / BO2 + 3H2
(36)
BH3OH + 3OH / BO2 + 3/2 H2 + 2H2O + 3e
(37)
BH3OH + 6OH / BO2 + 5H2O + 6e
(38)
BH4 + xOH / BO2 + (x 2)H2O + (4 ½x)H2 + xe
(39)
The formal reduction potential for the 8e BH4/BO2 redox
couple is >300 mV negative to the formal potential of the HER
(eqn (26)). It should therefore not be a surprise that the
reduction of H2O is thermodynamically favourable (a 1.64 V cell
potential is wider than the potential window of stability for H2O
on Pt). Operating DBHFCs at high current densities (lower cell
potential and higher anode potentials) will help to minimise
parasitic HER. There are reports that the addition of small
quantities of thiourea acts as a hydrolysis inhibitor.262
All of these factors reduce the number of e that are
extracted from each BH4 molecule (reduces fuel efficiency).
The HER is especially problematic in fuel cell stacks263 as H2
evolution in the early cells of the stack can affect the operation
of cells farthest away from the inlet (i.e. losses occur due to
uneven fuel distributions). The evolution of H2 gas must also be
controlled unless it is desired to produce H2 for oxidation at the
anode of a PEMFC (an indirect DBHFC).264 Therefore, n < 8e
oxidation of BH4 is generally unavoidable and OCVs will not
approach the theoretical maximum (due to a mixed potentials
from the presence of both BH4/BO2 and H2O/H2 redox
couples).
Pt, Pt-alloys, Ag, Au, Zn, Ni, Pd, Os, Cu, and hydrogen storage
alloy based catalysts have all been studied for BH4 oxidation.257
The common claim that Au catalysts oxidise BH4 via 8e
without competing hydrolysis reactions may not be universally
true, while Pt can actually achieve near full BH4 oxidation (e.g.
at low anode potentials);265 however, studies with Au and Pt may
be complicated by catalyst poisoning by BH4 oxidation intermediates (possibly BH3OH).266 Ag-based catalysts are less
active and the presence of oxidised surface oxide species
appears to be a prerequisite for borohydride oxidation.267 Pdbased catalysts can achieve higher peak power densities and
reduce the rate of H2 evolution.268 The need for a rational design
of binary anode catalysts, such as Au2Cu(111), has been
identied.269
In general for AAEM-based DBHFCs, peak power densities
up to 250 mW cm2 are reported (OCVs values tend to be in the
range 0.8–1.0 V).262b,270 Huang et al. argue that the use of KBH4
can give improved performances over NaBH4 in AAEM-DBHFCs
(lower KBH4 permeability and higher KOH-conductivity in the
PVA AAEMs).270a The highest AAEM-based DBHFC performance
reported to date is by Zhang et al. who achieved 321 mW cm2
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(at 700 mA cm2) at 40 C in a BH4/O2 fuel cell [anode supply ¼
NaBH4 (1 mol dm3) in aqueous NaOH (3 mol dm3) and Pt/C
catalysts used in both electrodes, AAEM ¼ a guanidinium-poly(silsesquioxane)-PTFE composite (IEC ¼ 1.14 meq. g1 and 65
mS cm1 at 60 C)].271
For comparison, Miley et al. achieved 680 mW cm2 (OCV of
1.95 V) in a Naon®-based NaBH4/H2O2 DBHFC stack at 60 C
with a Pd anode catalyst and an Au cathode catalyst (anode ¼
18% mass NaBH4 in alkali and cathode ¼ 17% mass H2O2 in
acid).272 The concept of using an alkali anode and an acid
cathode is a recurring theme273 but may not be relevant to
AAEM-containing DBHFCs. Liu et al. have achieved 663 mW
cm2 at 65 C in a BH4/O2 (i.e. non-peroxide) DBHFC containing a porous polymer bre membrane (PFM) separator and
using aqueous KBH4 (0.8 mol dm3)/KOH (6 mol dm3) as the
fuel feed [CoO cathode catalyst and LiNiO3 anode catalyst];274
the performance dropped to (a still respectable) 390 mW cm2
when Naon® NRE-212 was used instead of the PFM.
Mixed N and B fuel options. Another proposed concept
involved the use of an alkaline mixed BH4 + N2H4 anode feed
where a fuel cell containing AAEM (from Asahi Kasei Corporation) outperformed a CEM-containing analogue.275 The performance of the AAEM-containing fuel cells increased with
increasing hydrazine content and the cell supplied with
hydrazine only aqueous feed (15% mass hydrazine in 10% mass
NaOH) produced the highest performance (92 mW cm2 with
dry O2 cathode supply).
Alkaline direct ammonia borane (AB ¼ H3B)NH3) fuel cells
have also been proposed.250b,276 AB (also known as borazane) is a
water soluble white crystalline solid containing 19.5% mass
hydrogen (AB is a proposed hydrogen storage material), which
is stable at high pHs.277 Au and Ag catalysts again look promising for AB oxidation in alkali (MnO2 catalysts appear unaffected by AB so again has promise as a cathode catalyst).276e,278
The reactions (and side-reactions) involved in the oxidation of
AB in alkali (and for an overall AB-AFC) are given below:
BH3NH3 + 7OH / BO2 + NH3 + 5H2O + 6e
E ¼ 1.22 V vs. SHE
BH3NH3 + OH + 3/2 O2 / BO2 + NH3 + 2H2O
Ecell ¼ 1.62 V
(40)
(41)
BH3NH3 + 4OH / BO2 + NH3 + 3/2 H2 + 2H2O + 3e (42)
BH3NH3 + 2H2O / BO2 + NH4+ + 3H2
(43)
The latter non-electrochemical reaction (eqn (43)) is thermodynamically spontaneous and so it is important that catalysts that are active for AB oxidation but not AB hydrolysis. An
AB-AFC is less efficient than a BH4-AFC due to the lower
number of e extracted per fuel molecule (6 [or 3 if eqn (42) is
operating] vs. 8), despite similar Ecell values. Lu et al. rst
reported an AAEM-based fuel cell fuelled with AB,276e which
concluded that the 3e process (eqn (42)) was predominant
(along with some capacity loss via hydrolysis). More recently Xu
et al.276c reported that a direct AB fuel cell containing a
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Tokuyama AEI (IEC ¼ 2 meq. g1) and a Tokuyama 27 mm thick
AAEM (IEC ¼ 1.8 meq. g1) achieved an OCV of 0.9 V and a peak
power density of >110 mW cm2 at 45 C [Pt-based anode and
cathodes, anode feed containing AB (0.5 mol dm3) in aqueous
NaOH (2 mol dm3), humidied cathode feed]; however, H2
evolution was still a problem. In reality, and as expected from
the discussion above, the BH3 component of AB is more easily
oxidised than the NH3 component. Considering AB is expensive
to produce (or regenerate from BO2), this fuel may only be
suitable for niche applications.
Finally, other B-based fuel options exist but have not been
considered as a fuel option in AAEM-based fuel cells. This
includes the volatile liquid borazine [(BH)3(NH)3, a cyclic
molecule that is isoelectronic and isostructural with benzene].
Borazine is normally a trace PEMFC poison when AB is being
used as a H2 storage material and it can also polymerise into
polyborazylene “gums”.
The operation of carbonate-cycle APEFCs
Impacts and promise for carbonate anions. As discussed
earlier, a pressing issue facing the implementation of AAEMs
and AEIs for a whole host of applications are their chemical
stability in highly alkaline (OH) media. Most researchers have
responded to this limitation by designing speciality
membranes. Additionally, operation with air supplies to the
cathode is desired, which is problematic due to the potential for
carbonate formation in the electrolyte (eqn (1) and (2)).
Even though there is growing awareness that CO32 does not
generally have such a serious deleterious effect on APEFC performance,9e,152,279 HCO3, however, has a strong negative effect.43a,116a,280 HCO3 has a much larger hydration radius than OH
(ca. 4 vs. 3 Å respectively) with the same valence,281 which
signicantly reduces conductivity and device performance in
the presence of CO2.93 On the other hand, CO32 anions have
double the valence of OH, which means that despite its larger
hydration radius,281b there will be lesser effect on AAEM
conductivity.152 In addition, even commercial AEMs (that were
not developed specically for highly alkaline environments,
such as that found in APEFCs), and those that are not stable in
the presence of aqueous OH (such as a number of phosphonium exemplars), are far more stable in HCO3/CO32
compared to OH environments/forms.9e,75,279b
All of this suggests the possibly of an alternative high impact
(disruptive) approach: to abandon OH anions altogether and
transition to low temperature devices that use CO32 conduction (cycles). The deliberate utilisation of CO32 in low
temperature electrochemical systems has only been investigated since 2006, with the lion's share of the effort concentrated
on the development of a room temperature carbonate fuel cells
(RTCFC).282 However, there has been a recent but slow increase
in the amount of work being conducted to investigate alkaline
electrochemical devices that purposefully utilise CO32 anions
for energy generation (fuel cells), to facilitate new chemical
synthesis pathways, and for CO2 separation.279,283
Selective carbonate formation at the cathode. For the traditional ORR at the cathode (eqn (9)), it has been reported that the
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presence of CO2 and CO32/HCO3 does not noticeably impact
on the intrinsic electrochemical kinetics;279b,284 however, there
may be a negative impact on mass transport near the electrode
surface (that leads to reduced device performance). The loss of
performance has also been echoed in CO32-intended devices;
generally low current densities have been observed compared to
OH-based devices. This is hindered by the near non-existence
of CO32-focused materials development in the literature. For
example, only one catalyst (Ca2Ru2O7) has been reported in the
literature that can produce CO32 anions through a direct
electrochemical reduction with high selectivity (eqn (44) rather
than the reaction in eqn (9) that generates OH ions):283c,d
[RTCFC Cathode] O2 + 2CO2 + 4e / 2CO32
(44)
[RTCFC Anode] 2CO32 + H2 / H2O + CO2 + 4e
(45)
Such catalysts, including when doped with Bi, exhibit
intriguing behaviour in the presence of CO32 in aqueous
alkali.285 However, Ca2Ru2O7 catalyst still has many issues. Most
notably, it is hard to synthesise and it has a CO2 adsorption
strength that is too large (leads to an optimum CO2 concentration at the cathode of 10% mol, which yields a very low CO2
activity in the cell).
The direct formation of CO32 in these devices is important
since the competing indirect pathway (i.e. involving the chemical reaction between CO2 and electrochemically generated
OH) still involves OH desorption (implications for the durability of the AAEM and AEI). The indirect route also risks the
production of HCO3, which will lower the ionic conductivity of
the AAEM and AEI. However, researchers have also failed to
make AAEMs with the appropriate functionality to maintain the
anions as CO32. The equilibrium balance between OH, CO32
and HCO3 in the membrane is dictated by the effective pKa
values of the cationic functional groups. All existing AEMs have
effective pKa values that are either too high (leading to mostly
OH) or too low (leading to mostly HCO3). Thus, even if CO32
were produced with 100% selectivity at the catalyst, the lack of
CO32-specic membranes would cause the concentration of
CO32 to shi towards HCO3 and OH (+CO2) anyway. This
suggests that a concerted materials design effort is needed in
this area.
Carbonate as an oxidizing agent at the anode. It has been
shown that the HOR by CO32 (eqn (45)) is kinetically facile,279a
perhaps even more so than the HOR by OH (eqn (8)); however,
similar to the ORR, the presence of CO32 seems to have a
deleterious effect on the mass transport near the electrode
surface.286 Carbonate has also shown the ability to oxidise
CH3OH at a higher rate than OH anions on NiO catalysts.180 It
is thought that the enhanced kinetic rate is due the inherent
difference in how the two anions oxidise fuels: OH essentially
oxidises species by accepting protons whereas CO32 oxidises
species by abstracting an oxygen and donating it to hydrogen.
Taking advantage of that mechanism, there has been other
work outside of the fuel cell arena, where CO32 anions have
shown the ability to electrochemically activate methane at room
temperature (see later section on CO2 reduction
3158 | Energy Environ. Sci., 2014, 7, 3135–3191
electrolysers)283a,b unlike OH,287 opening new areas for research
on electrochemical synthesis that was simply not possible until
now. Although CO32 has shown initial promise as the reacting/
conducting anion in low temperature electrochemical devices, a
considerable amount of work remains to develop a solid
fundamental understanding of the reaction mechanisms that
are occurring and the materials requirements.
Hybrid AAEM/PEM fuel cells
As described in the previous sections, fuel cells and other
APEFCs operating at high pH using AAEM materials have
attracted attention due to their favourable operating parameters
with major advantages including the use non-noble metals at
the cathode and a wider range of fuel options at the anode.2,288
However, the lower ionic conductivity of AAEMs compared to
PEMs (such as Naon®) at lower RHs is a concern because it
may lower the performance.52 PEMFCs and APEFCs require
careful water management because water is consumed at the
cathode in APEFCs and water is needed for ion hydration (in
both). Eqn (8) and (9) show the reactions at an APEFC anode
and cathode, respectively, while eqn (6) and (7) give the acidic
PEMFC analogues. In both cases, the overall reaction is given in
eqn (10). The analogous anode reactions for methanol fuelled
systems are given by eqn (12)/(13) and (46):
CH3OH + H2O / CO2 + 6H+ + 6e
(46)
Water management is challenging in the PEMFCs because
water is produced at the cathode and needed at the anode (for
hydration of the proton and production of carbon dioxide in the
case of methanol). APEFCs are also challenged by water
management because water is consumed at the cathode (to
make the OH anions) and needed for hydration of the ions.
Water is produced at the anode for both H2 and methanol
fuelled AAEM-based fuel cells. Thus, signicant resources or
careful system design are required to recycle the water from one
electrode to the other for both PEM-based and AAEM-based fuel
cells. In both cases, more water is produced than consumed due
to the net production of water (eqn (10)).
Hybrid PEM/AAEM membranes and the junction potential.
Bipolar membranes are a combination of anion and cation
conducting materials where an ionic junction is formed and the
type of conducting ion changes at this materials junction.289
Such bipolar/hybrid membranes have been used in a wide range
of electrochemical devices such as those that involve CO2
separation via electrodialysis.290 Hybrid (bipolar) fuel cells can
be constructed using an AAEM anode and PEM cathode
(Fig. 5a), or PEM anode and AAEM cathode (Fig. 5b). The latter
conguration, where water is created at the junction (eqn (47)),
is of more interest for fuel cells because it takes advantage of the
high conductivity and established infrastructure of PEMs and
exploits the advantages of a high pH cathode. It also can provide
self-hydration within the membrane at the junction:
H+ + OH / H2O
(47)
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This contribution allows the use of non-Pt catalysts at the
cathode and an opportunity to generate the water at a junction
that can be located close to where the H2O is consumed at the
alkaline cathode.291 In principle, the PEM|AAEM junction can
be placed anywhere within the structure. The water created at
the PEM|AAEM junction is dual use and contributes to both
the self-hydration of the membrane and the ORR at the
cathode. Excess water can leave the system through either side
of the structure. Thus, this structure uses a PEM anode (eqn
(6)) and AAEM cathode (eqn (9)). The overall full cell reaction
(sum of eqn (6), (9) and (47)) is the same as for PEMFCs and
APEFCs (eqn (10)). In the case of a methanol fuelled system,
the acid anode is given by eqn (46) and the overall full cell
reaction at steady state is the same as for AAEM- or PEM-based
DMFCs.
The equilibrium cell potential (Ecell) for the hybrid (bipolar)
conguration needs to account for the reactions at each electrode and the junction potential (Ej) developed at the PEM|
AAEM interface:
Ecell ¼ ENernst + Ej ¼ EC EA + Ej
(48)
where ENernst is the difference between the cathode (EC) and the
anode (EA) potentials. Using the Nernst equation, the following
is obtained:
2
3
1
RT 4PO2 2 PH2 5 RT AEM PEM o
o
ln
ln aOH aHþ þEj (49)
þ
Ecell ¼ EC EA þ
2F
F
PH2 O
where EoC and EoA are the standard potentials for the cathode
and anode reactions, R is the ideal gas constant, T is the
absolute temperature, F is Faraday's constant, Px is the partial
pressure of gas x, and aM
z is the activity of ion z in membrane
M. At the PEM|AAEM interface, neutralisation of the mobile H+
in the PEM and OH within the AAEM occurs leaving behind
the xed charges bound to the polymer membranes.291c Neutralisation of the H+ and OH ions continues until the electrostatic attraction of the xed charges (counter ions bound to
the PEM or AAEM polymer backbones) holding the ions in
their respective membranes balances the diffusion across the
membrane. At this point, the junction is in thermal equilibrium and a junction potential is created by the separation of
xed charge across the junction. The junction potential is
given by eqn (50) and rearranged to eqn (51) recognising that
Kw ¼ aH+aOH:
!
aPEM
RT
AEM
PEM
Hþ
f
¼
(50)
Ej ¼ f
ln AEM
F
aHþ
Ej ¼ fAEM fPEM ¼
RT PEM AEM RT
ln aHþ aOH lnðKw Þ
F
F
(51)
where fM is the potential within phase M. In the hybrid, bipolar
fuel cell, the theoretical maximum cell potential at 25 C is the
same as in a PEMFC or APEFC (1.23 V). Even though the
Nernstian contribution (EC EA) in eqn (48) is not 1.23 V, the
difference between the Nernstian potential and cell potential is
exactly off-set by Ej (eqn (51)). The width of the junction region
This journal is © The Royal Society of Chemistry 2014
Details of hybrid membrane fuel cells comprising of: (a) high pH
AAEM anode + low pH PEM cathode; and (b) low pH PEM anode + high
pH AAEM cathode.
Fig. 5
(W) in the bipolar membrane is a function of the density of
charge in each of the two materials is given by:291c
23Ej 1
1
W¼
þ
q
Nþ N
1
2
1
23kT 1
1
Nþ N 2
ln
¼
þ
Kw
q2
Nþ N
(52)
where 3 is the dielectric constant, q is the elementary charge, k is
the Boltzmann constant and the density of xed charges (N+/)
can be expressed by the IEC within the polymer electrolytes
using:
N+/ ¼ NA IEC+/ rm
(53)
where NA is Avogadro's number and rm is the density of the
polymer electrolyte.
Hybrid fuel cell operation. The performance of the hybrid
cells and the self-humidication was demonstrated by operation at different relative humidity levels.292 The cell voltage was
recorded as a function of time for a hybrid PEManode/
AAEMcathode cell operating on H2 and O2 at 100 mA cm2 current
density at 600 C. The RH was increased from 0% to 100% in
increments of 25% every 24 h. The current remained nearly
constant with time and humidity. The current–voltage relationship for the cell was recorded at the end of each 24 h period
and is shown in Fig. 6(top). The current–voltage curves are
nearly identical at low current densities (<100 mA cm2) for
each RH condition. Interestingly, the performance was higher
at low RHs.
In situ a.c. electrochemical impedance spectroscopy was
used to help understand the change in cell performance with
relative humidity. For all humidity conditions, the impedance
spectra (600 mV cell voltage at 60 C) exhibited semi-circular
loops, as seen in Fig. 6(top). The high frequency x-axis intercepts (predominantly the MEA ohmic resistance derived mainly
from the ionic resistance of the membrane)293 showed that the
ionic resistance of the membrane was nearly constant over the
gas RH range RH 0–100%. However, the radius of the semicircular loop increased with RH. The difference between the xintercept values of the semi-circular responses at high and low
frequency is mainly governed by interfacial ORR kinetics, ionic
conductivity and diffusion limitations within the depletion
layer.293b Since the decrease in ionic conductivity of the PEM at
high humidity is not expected, diffusion limitations within the
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catalyst layers is a likely reason of the increased low frequency
resistances at higher RHs.
These results demonstrate that the water generated at the
PEM|AAEM interface maintains adequate hydration in the
hybrid membrane when the inlet gases are dry. Hydration of the
gas streams results in excess water within the membrane,
ooding of the electrodes, and limited O2 diffusion in the
cathode catalyst layer. This is a signicant result because the
performances of conventional PEMFCs (and APEFCs) oen have
to rely on fully humidied gas feeds. Hydration or wicking of
water (from one electrode to the other) can increase complexity
and lead to a loss in efficiency.
Beyond fuel cells, the hybrid (bipolar) structures could
contribute to more efficient water electrolysers, salt-splitting
technologies, electrochemical separators (such as CO2 pumps),
and solar-to-fuel applications. This is because the pH of each
electrode can be taken to extreme values and optimised for the
highest efficiency with the materials present. In addition, the
bipolar structures may improve permselectivities compared to
single ion conducting membranes because the two kinds of
ions migrate in opposite directions. One example of the
potential optimisation of the electrolyte based on the materials
present is the solar-to-fuels systems proposed by Spurgeon et al.
(Fig. 7).294 Sunlight is absorbed at each of the two photoelectrodes. p-type Si in a PEM environment was proposed for the
photocathode because it is cathodically stable in acidic
Fig. 6 Top: Cell voltage vs. current density curves at 60 C for a hybrid
PEManode/AAEMcathode fuel cell with RH levels ranging from 0% to 100%
(symmetrical for both H2 and O2 gas supplies).292 Bottom: Corresponding in situ a.c. impendance spectra at Vcell ¼ 600 mV.
3160 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
media.295 The photoanode may be a metal oxide semiconductor,
which could be unstable in an acidic environment but stable at
high pH.296 Irradiation with sunlight produces H2 gas at the
PEM cathode and O2 at the AAEM anode. The direction of ion
ow is opposite to that shown in Fig. 5b and water is split into
H+ and OH ions at the junction (Fig. 5a). A hygroscopic junction between the PEM and AAEM materials is desired due to
water consumption there.
Future needs and directions. The bipolar PEM|AAEM
membrane may potentially address some of the critical issues
faced by PEMFCS, especially problems of platinum utilisation
and water management. There are many challenges facing
hybrid (bipolar) membranes that are used at extreme pH values.
The rst is the formation of a tightly bonded and stable PEM|
AAEM junction, the location of an abrupt conductivity change
that must withstand the high internal pressure due to the
formation or consumption of water at that location. Solutions to
this may include the bonding of two prefabricated membranes
(i.e. a PEM and an AAEM) with a third (deposited) layer to
covalently or ionically bond the two lms together. Another
approach is to form a single layer and chemically convert one
side into a cation conducting material and the other side into an
anion conducting material (i.e. formation of a chemical interface without a “physical/mechanical” interface). The initial
homogenous single layer may be an ionic precursor (e.g.
contains alkyl halide functionality that can be converted to both
cation- and anion-exchange groups in different regions) or may
already contain ionic conducting functionality (such as sulfonic
acid group) that is then subsequently converted (partially) to an
anion conducting group.
The design and analysis of an efficient, low-loss, bipolar
structure is important. Charge neutralisation occurs at the
PEM|AAEM junction resulting in a drop in conductivity. The
width of the junction region (eqn (52)), is important because the
high resistance in this region will result in ohmic losses. The
placement of the junction within the electrochemical device is
important because water will either be generated or consumed
there (depending on the type of device).289 The location of the
junction within the membrane should consider the transport of
water within the system. In the case of the fuel cell described
above, this includes the consumption of water at one electrode
Fig. 7 A solar-to-fuel system that could incorporate bipolar
membranes.294
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and the efficient removal of excess water. The water balance is a
function of the RH of the gas feed streams at the two electrodes.
The analysis of the width of the junction, location of ions, and
impact of conductivity across the junction is challenging. There
are few analytical techniques with adequate spatial resolution,
chemical specicity, and sensitivity to address the problem.
High surface area, non-planar junctions may be of value to
minimise the ohmic losses at the junction (i.e. where the real
surface area is greater than the supercial surface area) and may
provide additional or alternative locations for water generation/
consumption (depending on changes in relative humidity of the
feed streams).
The need for carbon free catalyst supports at high pHs
Carbon is the favoured material for many fuel cell components
for PEMFC and APEFCs.297 However, carbon is thermodynamically unstable across virtually the entire potential range in
which fuel cells operate. Under standard conditions (pH ¼ 0,
298 K), carbon is thermodynamically stable only over the
potential range 0.05–0.15 V (vs. SHE) as depicted in the relevant Pourbaix diagram (Fig. 8). The fact that carbon has become
such an important material for catalyst supports, reactant
transport layers, and bipolar plates is related to its relatively low
cost, moderate electrical conductivity, and signicant kinetic
barrier to corrosion. The barrier to corrosion comes about
because carbon, once oxidised, has a preferred oxidation state
of +4 (i.e. carbon oxidation requires 4e) and the ultimate
product is an oxide, the formation of which is hindered by large
activation energy barriers in acidic environments. Furthermore,
there are no intermediate states of oxidised carbon that are
soluble or produced in signicant quantities along the reaction
pathways leading to CO2 (e.g. methanol, formaldehyde, and
formic acid are not produced in appreciable quantities):
C + 2H2O # CO2 + 4H+ + 4e E ¼ 0.118 V vs. SHE
(54)
C + 6OH # CO32 + 3H2O + 4e E ¼ 0.854 V vs. SHE
(55)
The corrosion properties of carbon are relatively well
appreciated in PEMFCs with the issue being viewed as a concern
for the longevity (durability) of these systems. The most extreme
events that challenge carbon stability are faced during start-up
and shutdown procedures. Fuel starvation events can lead to
the cathode potentials rising up to potentials of 1.2–1.5 V (vs.
RHE).298
However, carbon shows even less thermodynamic stability
under alkaline conditions (as can be seen from Fig. 8); this is in
addition to the fact that carbons are active towards the undesirable n ¼ 2e ORR at high pHs. This is not the end of the
matter either as the kinetics for carbon oxidation actually
accelerates in alkaline environments due to OH anions being
excellent nucleophiles. Indeed, carbon has been suggested (as
far back as 1896 by Jacques) as anode fuel in molten NaOH
carbon air batteries;299 such systems were constructed that
provided up to 1.5 kW of power.300 More recently, the prospect
This journal is © The Royal Society of Chemistry 2014
Fig. 8 Pourbaix diagram for carbon at 298 K showing the hatched
domain of stability. Predominate gaseous (italicised) and solution
phase species (non-italicised) are shown in the domains where the
activity (or partial pressure) is 106. (a) Corresponds to the lower
stability window of water (HER) and (b) corresponds to the upper
stability limit of water (oxygen evolution reaction, OER).
of using carbon as an anode fuel in a molten AFC has received
further attention.301
As might be expected from the Pourbaix diagram, the
degradation of carbon is liable to be more extreme at high pHs
due to the greater thermodynamic stability of CO32 compared
to CO2. Decomposition of fuel cell components in aqueous
electrolyte AFCs was moderately well studied in the 1970s and
80s. Long-term corrosion rate studies of Black Pearls 2000 and
Vulcan XC 72R in aqueous KOH (12 mol dm3) at 80 C for
>1500 h showed that the carbons are more severely attacked (cf.
in aqueous H3PO4 under the same conditions): aer 2300 h, the
BET surface area of the Vulcan XC 72R reduced from 225 to 125
m2 g1.302 In a more recent study, a carbon composite bipolar
plate material (30% polymer ller, 5% XC-72R C black, 5%
Toray carbon bres, and 60% graphite powder) was tested
under simulated APEFC conditions [aqueous NaOH (1 mol
dm3)] and compared to PEMFC conditions [aqueous H2SO4 (1
mol dm3)]. An 18 fold increase of corrosion current was
observed with the alkaline conditions.303
Alternatives to carbon. Because of their relative stability in
alkaline environments, two metals have been examined extensively as GDL layers and supports for use in AFCs: Ni and Ag.
The original aqueous electrolyte AFC developed by Bacon for the
US space programme utilised porous Ni electrodes with dual
porosity, which was later modied to contain a lithium-doped
nickel oxide cathode (to reduce corrosion problems).300 Others
have noted problems with using Ni in the cathode, predominantly due to the poor conductivity of the oxide that forms; this
leads to large iR loses aer only a few days of operation. This can
be ameliorated by either plating the Ni with Ag or by using a Agonly cathode.302
Despite the above, the current preferred material for reactant
transport layers in APEFCs is carbon as the issues with carbon
corrosion in the reactant transport layers are normally minor
(as the electrolyte is not mobile and hence does not tend to wet
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the transport layer). As the transport layer is not exposed to
overly harsh alkaline conditions, carbon is suitable for such
application. However, other alternatives are possible and may
be borrowed from the eld of liquid (aqueous) AFCs: e.g. see the
review by Bidault et al.2q For example Nickel coated PTFE shows
an electronic conductivity of 300 S m1;304 Ni foam may also be a
suitable alternative.305
One particularly interesting area is the production of
combined catalyst/reactant transport layers utilising a porous
Ag membrane.306 As Ag has the highest electrical and thermal
conductivity of any metal, its use in the reactant transport layers
may alleviate some of the issues found in high performance fuel
cells caused by local heating effects. A 50 mm thick Ag reactant
transport layer with a porosity of 60% yielded a sheet resistance
(0.8 mU ,1 [where , indicates square, a commonly used unit
for sheet resistances measured using 4-point methods]) that
was 400 lower than that of standard carbon-based reactant
transport material (Toray TGP-060, 294 mU ,1). Hence, it is
not necessary to keep the gas supply channels so narrow.
Indeed, when using the Ag gas transport layer mentioned above,
channels could be 20 mm wide without incurring larger ohmic
loses compared to the use of a carbon based gas transport layer
with 1 mm wide channels. The cost of the Ag reactant transport
layer is approximately 3 the cost of the Toray carbon-paper
and so this is not a critical issue. A major benet of using a Ag
reactant transport medium is that it also functions as the
catalyst, producing impressive electrochemical performances
both in the absence and in the presence of additional catalyst.307
Yan et al. has also advocated support-less Ag nanowire ORR
catalysts.308
AAEMs in alkaline electrolysers3
H2 electrolysers containing AAEMs and/or AEIs
H2 production from water electrolysis. H2 can be produced
using chemical, electrochemical, catalytic, thermal or biological
processes.309 Interest in H2 has increased because of its potential use as a fuel produced from renewable and sustainable
resources.310 The current production of H2 is dominated by
catalytic steam reforming of methane,which produces ca. 95%
of the H2 used worldwide. The remaining commercial production of H2 is mainly via electrolysis of water (a convenient and
simple route). Electrolysers produce very high purity H2 for use
in several applications (e.g. semiconductor manufacture,
hydrogenation of food products, and the production and
rening of high purity metals).309 Most commercial electrolysers
are based on alkaline electrolysis and operate at current
densities in the region of 1000–3000 A m2 and contain aqueous
electrolytes of approximately 30% mass KOH (gives the
maximum ionic conductivity of 1.5 S cm1 at 80 C).311 Obviously, carbon-based materials (e.g. in the electrodes and bipolar
plates) cannot generally be used for alkali or PEM-based electrolysers, as carbon corrosion occurs at the potentials being
applied (see previous section).312
For alkaline electrolysers, the individual electrode reaction
that produces H2 (HER)313 at the cathode is given in eqn (26),
3162 | Energy Environ. Sci., 2014, 7, 3135–3191
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while the O2 producing reaction (oxygen evolution reaction,
OER)121 at the anode is given by:
Anode (OER): 4OH / O2 + 2H2O + 4e
E ¼ 0.40 vs. SHE
(56)
In the cell, a separator is used to keep the H2 gas isolated
from the O2 gas (to avoid formation of a potentially explosive
mixture). As with APEFCs (Fig. 9a), an alkaline electrolyte
enables the use of low cost non-PGM catalysts, such as Ni,
which helps to keep capital costs of the cells relatively low.
With the use of acid electrolytes, electrolysers typically use
solid polymer electrolyte (SPE, e.g. a PEM) and not an aqueous
electrolyte (e.g. sulphuric acid).314 In acid electrolysis precious
metal catalysts are used to achieve efficient electrolysis, which
generally means that high rates of H2 production (per unit area
of electrode) are required to minimise the capital costs of the
cells. There are clear similarities between the cells used for
water electrolysis and those used in PEMFCs because the
central component, the polymer electrolyte membrane, is
essentially the same for both. Consequently, technological
developments in polymer electrolyte based fuel cells can oen
be transferred to SPE electrolysers.
Why AAEMs and AEIs in electrolysers? A traditional alkaline
electrolyser uses a porous diaphragm to isolate the O2 and H2
gases and to prevent intermixing of the catholyte and anolyte
(two-phase electrolytes) in order to obtain high gas purities and
high current efficiencies.310 The diaphragm ideally need to
prevent the formation of a gas bubble “curtain” at the front side
of the electrodes (when pressing the electrodes onto the elastic
diaphragm) to ensure low ohmic and contact resistances.
Separators for industrial alkaline water electrolysers can be
made from either inorganic or organic materials. For low
temperature electrolysers, the separator can be nickel oxide,
asbestos, or a polymer. The asbestos diaphragm, widely used in
alkaline water electrolysis,315 has a high resistance, is carcinogenic (i.e. asbestosis), and is unsuitable for use above 100 C.
Diaphragm materials have also been made from polyantimonic
acid316 and a polysulfone/zirconium oxide (Zirfon®) composite
membrane317 and both are relatively thick (1–2 mm).
Replacing the separator/diaphragms mentioned above with
an IEM can offer advantages such as reduced gas crossover and
Fig. 9 Comparison of: (a) a H2/O2 fuel cell containing an AAEM
[APEFC] with (b) an alkaline electrolyser containing an AAEM/HEM
[APEE].
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area resistances (especially when using thin membranes).
Applying AAEMs to electrolysers (Fig. 9b) provides the opportunity to consolidate the advantages of both types of traditional
electrolysers. The costs will be reduced with the use of the lower
cost electrode materials and catalysts that are used in alkaline
electrolysers, while the AAEM electrolyser (APEE) would not be
affected by the presence of cationic species (present in the feedwater). The latter is a major issue with PEM-based electrolysers:
a major reason for gradual deterioration in their performances
relates to the cations binding to the proton conducting
(exchange) sites of the PEM (which reduces its conductivity).318
In operation, PEM-based electrolysers are fed with pure water at
the anode which decomposes to O2 and H+ cations; the latter
pass through the PEM and are subsequently converted to H2
gas. The absence of a corrosive (aqueous/liquid) electrolyte is
one of the major features of SPE-type water electrolysers, which
adds to the simplicity of operation (and reduced costs of
components such as bipolar plates).
The central component of the electrolyser is an MEA (similar
to PEMFCs and APEFCs). The IEM serves as the ion (but not
electronically) conducting electrolyte and also as the separator
for H2 and O2 gases. Ideal electrodes require the following
attributes:
(a) Good electronic conductors;
(b) High structural integrity;
(c) Corrosion resistance with the electrolyte being used and
at operating potentials that are appropriate for the cathode
(reducing) and the anode (oxidising);
(d) Contain high performance electrocatalysts for both the
HER and OER;
(e) High (reaction assessable) surface areas to facilitate high
current densities and/or H2 production rates;
(f) Stable performance over extended periods of operation
(both on and off load).
SPE electrolyser cells require an intimate contact between
the phases of the MEA to achieve: optimal ionic and electronic
conductivities, high active surface areas of the catalysts, and
rapid gas release. High electrocatalyst specic areas are
required to decrease overpotentials and to avoid the appearance
of hot spots (that drastically shorten the life span of the MEA).319
For SPE water electrolysers, both the anode and cathode electrocatalysts are deposited as thin (several mm thick) coherent
layers that are bonded to either side of the IEM.
The main challenge regarding the widespread use of H2 in
small to medium size applications is cost reduction (needed to
increase the commercial appeal of such H2 generators). Lowprice domestic electrolysers can be realised through high
production/sales volumes, but this will only occur aer
economical, efficient, and durable prototypes have been
demonstrated. Adopting AAEM-based technology can open up
opportunities for low cost electrolysers systems with: low
membrane, catalyst (Pt-free), and bipolar plate manufacturing
costs; higher energy efficiencies (towards “zero gap”);320
durable, long life operation (unlike with APEFCs [fed with gas
supplies of various RHs], the AAEMs are in an environment where
they remain fully hydrated in APEEs); exibility to respond to
dynamic load operation; and compact system integration and
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Energy & Environmental Science
control. The use of AAEMs may also facilitate the simultaneous
production of H2 and useful chemicals (e.g. potassium acetate)
when electrolysing aqueous alcohol solutions using only 1/3 of
the energy required by traditional H2/O2 electrolysers.321
Performance of AAEM-based electrolysers (APEE). Research
into the development of AAEMs, historically for use in APEFCs,
has produced materials that are of interest for use in APEEs. In
comparison to proton-conducting (PEM-based) electrolysers,
the amount of research conducted to date on AAEM-based SPEelectrolysers is, however, very small (see Fig. 11 in ref. 322).
Below is a quick review of key studies.
A solid state water electrolyser has recently been reported
that achieved 399 mA cm2 at a cell voltage of 1.8 V and nearly
1000 mA cm2 at 2.0 V when using Tokuyama A-201 AAEM,
Tokuyama AS-4 AEI, and aqueous KOH (1 mol dm3) solutions
at 50 C.323 This system, however, used high loadings of
precious metal catalysts (RuO2 at the anode and Pt at the
cathode). An alkaline electrolyser that used nickel iron oxide
coated anodes, Pt cathodes, and a developmental AAEM (ITM
Power, 160 mm thick) with aqueous KOH (4 mol dm3) solutions
at 60 C has been reported to achieve a cell voltage of 2.12 V at
1000 mA cm2.324 The use of KOH electrolyte is seen as important for achieving performances that approach those of Naon®
PEM-base systems where cell voltages between 1.6–1. 7 V are
possible at 1000 mA cm2. The use of NiMo and RuO2 coatings
on nickel or stainless steel micro-meshes have been examined
as electrocatalysts for HER in conditions similar to those in
“zero gap alkaline water electrolysers”.325 The NiMo and RuO2
coatings gave performances that were comparable to Pt and also
stable over 10 d of electrolysis; the performance of an electrolyser containing an AAEM (ITM Power, 160 mm thick) and a NiFe
(OH)2 coated anode (OER) with aqueous KOH (4 mol dm3) was
2.1 V at a current density of 1 A cm2. Jang et al. report ca. 150
mA cm2 at 1.9 V with aqueous KOH (1 mol dm3) feeds with
MEAs containing a Tokuyama A201 AAEM and electrodes consisting of Ni that is electrodeposited on carbon papers with very
low Ni loadings (8.5 mgNi cm2).322
Recent work at Newcastle University has used a methylated
melamine quaternised graed poly(vinylbenzyl chloride) AAEM
in an APEE containing a Cu0.7Co2.3O4 OER catalyst (Fig. 10);326 1
A cm2 was achieved at a voltage of 1.8 V in aqueous KOH (1 mol
dm3) at 25 C. In order to develop APEEs, a polymethacrylatebased OH conducting AEI ionomer binder (conductivity ¼ 59
mS cm1 at 50 C) was synthesised.327,328
A very recent study has reported the use of a dilute aqueous
K2CO3 solution in conjunction with an AAEM (A-201, from
Tokuyama Corporation).329 The MEA was based around low-cost
transition-metal catalysts. The HER and OER catalysts were
commercially available materials (manufactured by Acta SpA)
and based on Ni/(CeO2–La2O3)/C and CuCoOx mixed oxides
respectively. This system exhibited a similar performance to a
PGM-catalyst benchmark. The best performance reported at 43
C was a cell voltage of <1.95 at 500 mA cm2 (using 7.4 mg
cm2 of cathode catalyst). This approach uses a CO2 tolerant
and less aggressive alkaline electrolyte (i.e. no OH anions).
This again highlights that CO32 electrolyte systems warrant
further detailed studies.
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Metal-hydroxide-free systems. An early development with a
metal-hydroxide-free system (using only de-ionised water) was
reported in 2012 by Zhuang et al. where an electrolyser with
non-Pt electrodes was reported to achieve a current density of
400 mA cm2 with a cell voltage of 1.8–1.85 V at 70 C.330 The
AAEM was a self-crosslinking QA polysulfone (70 mm thickness,
ionic conductivity >0.01 S cm1), while the anode consisted of
40 mg cm2 Ni–Mo on a Ni foam current collector and the
cathode was Ni–Fe-based. A more recent study by Ramani et al.
also reported an APEE that produced H2 from ultrapure
water;331 400 mA cm2 was achieved at 1.80 V at 50 C with the
use of a QA polysulfone membrane, a Pt black HER catalyst, and
a high performance lead ruthenate pyrochlore OER catalyst.
This study is the rst to report on the performance losses of an
AAEM-containing APEE. Short-term degradation was due to CO2
intrusion into the system (and was easily remedied), whilst
longer-term losses were due to irreversible AAEM degradation
(polysulfone backbone hydrolysis).
Reversible water electrolysers containing AAEMs. The device
that combines a water electrolyser with a fuel cell is oen called
a regenerative fuel cell (RFC). If a water electrolyser also works
as a fuel cell (once H2 and O2 are supplied back to corresponding electrodes) it is oen called a unitized regenerative
fuel cell (URFC).332 Such devices may also be called reversible
water electrolysers, since they exhibit both functions of storing
electricity into chemical energy as electrolysers and then release
chemical energy back to electricity in the reversed electrochemical process. However, most URFC are based on acidic
PFSA ionomers and expensive noble metal catalysts; corrosion
resistant materials are necessary for good stability and cycle
life.333 For example in the bifunctional oxygen electrode, the
catalysts are oen based on expensive Pt with IrO2.333 However
in alkali, other options are available such as perovskite-doped
MnO2 for use as a bifunctional ORR/OER catalyst.334
It is of increasing interest to employ AAEMs into RFCs where
non-precious-metal catalysts can be used in the electrodes. A
recent study used a pore-lling AAEM made using porous PTFE
lled with a QA polymethacrylate AEI.335 The composite
membrane exhibited a lower swelling ratio (thickness and area
Fig. 10 Electrolyser performance with a OH ion conducting AAEM
and a Cu0.7Co2.3O4 cobalt oxide OER catalyst (circles) [cf. squares ¼
Co3O4 catalyst data]. T ¼ 25 C and aqueous KOH (1 mol dm3)
electrolyte at both the anode and cathode.326
3164 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
variation), stronger tensile strength, but lower ionic conductivity compared to a polymethacrylate-only AAEM. However, the
composite membrane was ultra-thin and therefore exhibited a
lower in situ MEA ionic (area) resistance and improved current
densities. In fuel cell mode, the peak power densities were 0.11
and 0.16 W cm2 at 20 and 45 C respectively. In water electrolyser mode, cell voltages at a current density of 100 mA cm2
were 1.61 V and 1.52 V at 22 and 50 C respectively, while the
degradation rate was only 40 mV h1 at a current density of 100
mA cm2 at 22 C.
Research challenges for AAEM-based water electrolysers.
PEM-based SPE water electrolysers typically have performances
of cell voltages <2.0 V at current densities >1000 m A cm2 (ca.
80 C) for extended operation over many thousands of hours.
For AAEM-based technology to be attractive, similar performances but with lower cost materials will be required. Development of such electrolysers will require research into several
key factors (many of which have overlaps to APEFC research
challenges):
(a) Efficient OER and HER catalysts with low activation
overpotentials and containing new catalyst structures or metal
alloys (resulting in lower noble metal loadings);
(b) AAEM with improved conductivities and alkali stabilities,
low gas crossover, and high mechanical stabilities;
(c) High performance AEIs for use in the catalyst layers;
These basic components are needed to fabricate the MEAs. A
key challenge is to bond high surface area catalysts, using an
AEI, to either the AAEM or a suitable metal supporting electrode
substrate. The MEAs must enable efficient gas release to prevent
gas bubble adhesion blocking the catalyst surface (and avoid
undesirable increases in polarisations in the cell). Thus the
surface of the MEA and AAEM should be hydrophilic. The
development of AAEM-electrolyser technology can benet from
existing research and know-how in the eld of electrocatalysts
for electrolysers containing liquid electrolytes.
Cathodes (HER). In alkaline water electrolysis, the cathode
material is typically either steel or nickel, which may be activated. Ni shows an initial high HER electrocatalytic activity but
experiences deactivation (that typically manifests itself as an
increase in HER overpotential at a constant current). High
surface area porous coatings of Ni, Ni–Co, Ni–Mo, and active
Ni–Fe layers on mild steel substrates have been developed with
various roughness factors (e.g. 2000 for Ni and 4000 for Ni–
Co).336 The HER overpotentials for these coatings were 100 and
90 mV respectively (ca. 135 mA cm2) at 70 C. H2 (adsorption)
storage alloys have been investigated for the HER and show a
pronounced improvement in HER kinetics. Ni–Mo HER catalysts have been reported to achieve 700 mA cm2 at an overpotential of 150 mV in in situ testing at 70 C.337 Ti2Ni alloy
exhibited a low hydrogen evolution overpotential (ca. 60 mV at
70 C in 30% mass aqueous KOH) and a very good stability
under dynamic operating conditions.338 The HER mechanism
on Ni–LaNi5 and Ni–MmNi3.4Co0.8Al0.8 materials were in
accordance with the Volmer–Heyrovsky mechanism.339 State-ofthe-art electrodes for HER based on RuO2 particles co-deposited
with Ni onto Ni supports have also been developed.340 Activity
was enhanced with the use of only relatively low amounts of
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RuO2 (in the Ni deposit) and the performance was stable under
conditions of constant and intermittent electrolysis. The
enhanced electrocatalytic activity of the cathodes can be mainly
ascribable to increased number of active sites (and/or the RuO2
content). Various oxides have been investigated for alkaline
water electrolysis. For example, La0.6Sr0.4CoO3 exhibited high
activity and good chemical stability for both OER and HER with
overpotentials for H2 formation similar to that of Pt. The anodic
OER overpotential of La0.6Sr0.4CoO3 was found to be much
smaller than those of Pt.341
Anodes (OER). In general, the anode must be stable at OER
potentials and at open-circuit. Ni has a high corrosion resistance at positive potentials in alkaline electrolytes and the
efficiency of OER of Ni is among the highest for the metals.
Because of the high price of nickel (>$13 000 per tonne),342 the
anodes are not, as a rule, manufactured from solid Ni but are
based on an electrolytically deposited non-porous nickel coatings. Increasing the electrode surface area (i.e. roughness factor
– dened as the ratio of its real surface area to its apparent, or
geometric, area) of Ni anodes have been achieved, for example,
by sintering ne Ni powders prepared from Ni(CO)4 decomposition.343 Coated electrodes (steel and Ni) can be made by
applying particulate metal coatings to the metal substrates.
Applying paint-like suspensions of metal particles can yield
uniform coatings of controlled thickness. Use of mild steel as an
anode substrate requires the steel surface to be protected from
corrosion (during OER) using fabrication methods such as
alloying.344 Raney Ni has been used to produce high surface area
anode coatings. Raney Ni is made by alloying Ni with metals
such as Al or Zn, which are subsequently leached out in alkaline
electrolyte yielding the desired high surface area structures with
high electrochemical activities.345
Several material types have been studied for alkaline electrolysers with decreased overpotentials, many of which are
metal oxide electrocatalysts for the OER in alkali. Catalytically
active materials such as NiCo2O4 have been applied.346 Catalysts
such as NiCo2O4, Li-doped Co3O4,347 and perovskite La1xSrxCoO3 types (and substituted variants) have been tested extensively.348 Mixed Cu–Fe–Mo oxide OER catalysts have also been
studied.349 Further research of this type may lead to important
catalyst modications or the development of entirely new
catalyst systems. Excellent OER activities have been reported
with oxides having the pyrochlore structure (described by the
general formula A2[B2xAx]O7y, where A ¼ Pb or Bi, B ¼ Ru or
Ir, O < x < 1, and O < y < 0.5). A typical Pb2[Ru2xPbx]O6.5 catalyst
evolved oxygen at an overpotential of only 120 mV at a current
density of 100 mA cm2 (and exhibited lower overpotentials
than Pt black, RuO2, or NiCo2O4).350 These Pb/Ru electrocatalysts are a reasonable precious-metal-based OER catalyst
option for alkaline electrolysis as ruthenium metal is relatively
inexpensive (ca. 10% of that of Pt).
Non-H2 electrolysers containing AAEMs/AEIs and involving
CO2 reduction
As discussed above, low temperature CO32 electrochemical
systems are in their technological infancy but there are
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increasing reports of electrochemical cells that involve CO32
reactions such as a CEM system for electrolysis of Na2CO3 and
NaHCO3 for the production of NaOH.351 The immature state of
development of room temperature AEM-based CO32 systems
makes them a potentially high impact area that is ripe for rapid
growth. Perhaps the best evidence for this is preliminary work
with low temperature carbonate electrochemical devices related
to methane activation to various high value products including
CO or methanol (Fig. 11):283a,352
2CH4 + 2CO32 / 2CH3OH + 2CO2 + 4e
(57)
This synthesis process involves a CO2 cycle when in
conjunction with the CO32 generating reaction in eqn (44)
(rather than conversion of all of the CO2 into products) and is
theoretically galvanic; in reality, with the overpotentials involved,
it will proceed via a low overpotential [driven] electrolytic process.
CO2 emissions make the greatest contribution to greenhouse
gases. Processes which convert CO2 to useful products are thus
desirable from a perspective of sustainability, and environmental protection. This type of system falls into the category of
Carbon Dioxide Utilisation (CDU)353 as opposed to Carbon
Capture and Storage (although OH containing polymer electrolytes have also been proposed for reversible CO2 capture).354
Overall in the production of chemicals from CO2, energy is
required to break the C]O bond to produce various chemicals
(the conversion of CO2 into CO32 is a rare example where the
conversion of CO2 into something else is thermodynamically
“downhill”). The electrochemical reduction of CO2 is the
conversion of CO2 to a more reduced chemical species using
electrical energy. Electrochemical reduction of CO2 is considered a possible means to produce chemicals or fuels from CO2,
making it a feedstock for the chemical industry.355 In prior
studies, direct electrochemical reduction of CO2 in low
temperature electrolysis has led to production of formic acid,
methanol, hydrocarbons and oxalic acid. Such transformations
make the electroreduction of CO2 of interest in a carbon energy
cycle.356 However, such low temperature electrochemical
reduction requires a relatively large amount of electrical energy.
No direct electrochemical CO2 reduction process has been
successfully commercialized, although academic and commercial efforts continue using a variety of homogenous and
heterogeneous catalysts. Generally speaking, the processes
developed to date either have poor thermodynamic efficiencies
high overpotentials, low current efficiencies, low selectivity,
slow kinetics, and/or poor stability. An exception to this is the
production of formate on carbon electrodes at high pH. Det
Norske Veritas and the Mantra Venture Group are both developing systems based on Sn cathodes that allow for the conversion of CO2 to formate:357
CO2 + H2O + 2e / HCOO + OH E = 1.05 V vs. SHE
(58)
The process is believed to use two phase electrochemical
reactors with Sn-based cathodes. The equilibrium potentials for
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Fig. 11 Schematic comparison of (a) an APEFC and (b) a CO32 cycle
reactor for the partial oxidation of methane into methanol (refer to eqn
(44) and (57) for desired cell reactions).
CO2 reduction are not too different to that for H2 evolution. This
however hides the fact that its reduction does not occur easily
and at much more negative potentials than the equilibrium
values.
Previous research on electrochemical CO2 reduction in
aqueous solutions has evaluated a wide range of metals.358 “1st
group” metals (such as In, Sn, Hg, and Pb) selectively form
formic acid/formate. “2nd group” metals (such as Zn, Au, and
Ag) form CO as a major product, while Ag, Au, In, Zn, and Sn can
produce CO and carbonate. Ni, Pd and Pt form CO selectively.
For example, a set-up involving an AEM was used to investigate
the electrochemical reduction of CO2 to C2+ products on Cu
surfaces.359,360 All of this suggests that the application of AAEMs
and AEIs into low temperature electrolyser systems involving
the electrochemical reduction of CO2 (rather than the more
well-known high temperature solid oxide and solid proton
conductor CO2 electrolyser systems)361 may have a major
impact.
AAEMs in alkaline batteries
Metal–air batteries (e.g. Al-, Zn-, Mg-, and Fe–air),362 and other
battery types such as Ni-MH (MH ¼ metal hydride) and Ni–Zn
cells, oen contain alkaline electrolytes and can have higher
energy densities and capacities compared to other batteries
such as Li-ion batteries. Catalysts such as Ag-MnOx/C can be
used for the O2 reactions when air electrodes are employed
along with alkaline electrolytes.363 In the last decade, there have
been a small number of reports of the use of AAEMs in such
batteries. One of the motivations for using an alkaline solid
polymer electrolyte is to prevent undesirable changes in the
electrodes (such as dendritic growth at the anode [growing
towards the positive electrode] in Ni–Zn cells).364 AEMs have
even been reported in Li–O2 batteries (to suppress direct LiOH
deposition in the air electrodes).365 As with APEFCs/APEEs, the
membranes need to have high ionic conduction and high alkali
stabilities.
Early examples of the use of solid polymers electrolyte in
alkaline primary and secondary batteries were reported by
Fauvarque et al. who used KOH-doped poly(ethylene oxide)
polymers and copolymers.366 Other examples include the work
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by Yang et al. who used a KOH-doped PVA-based copolymers in
a primary solid-state Zn–air battery and a Ni–MH battery,367
while Arof et al. similarly used a KOH-doped PVA electrolyte in a
Ni–Zn cell.364 A recent study investigated the use of KOH-doped
poly(acrylic acid) polymer electrolyte for all-solid-state Al–air
batteries.368 However, these are still metal-cation-containing
(KOH-based) systems.
Yasuda et al. investigated a reversible air electrode concept,
containing an AAEM with covalently bound cationic groups, for
use in secondary air batteries;369 the AAEM was a hydrocarbonbased QA-type (IEC ¼ 1.4 meq. g1, thickness ¼ 27 mm). The
rational for the use of such an AAEM-based air electrode was to
reduce ORR/OER performance losses by: blocking the cations
from penetrating into the air electrode (reduces carbonate
precipitation), reducing the penetration of the alkaline solution
into the air electrode, and preventing neutralisation of the
alkaline solution via the CO2 in the air electrode supply. A Pt–Ir
catalyst provided reduced overpotentials in the air–electrode
and the concept did exhibit a reduced (negative) inuence from
the presence of CO2.
AEMs in redox flow batteries (RFB)2a,4
IEMs play an important role as separators in some types of
RFBs4c,d,370,371 where a high degree of reactant isolation is
required between the anolyte and catholyte compartments.
Microporous separators can be used in RFBs to provide a barrier
between the two liquid streams (since the anolyte and catholyte
usually contain high concentrations of acids, bases, or other
electrolytes to facilitate ionic conductivity). However, while
being generally inexpensive and having low ionic resistances
when immersed in concentrated electrolyte solutions, microporous separators are prone to high crossover of electroactive
species that result in cell performance loses and severe battery
capacity fade.372
Thus, there has been a concerted effort to apply highconductivity, low crossover IEMs in a number of different RFB
systems. IEMs have been investigated for use in iron/chromium,
hydrogen/bromine, vanadium/bromine, non-aqueous,373 and
other types of RFB chemistries.370 Naon® and other peruorinated IEM variants have been the most heavily-studied
IEM in RFBs and most new membrane research (beyond
Naon®) has centred on all-vanadium redox ow batteries
(VRFB). In VRFBs, vanadium cations (in various oxidation
states) are used to reversibly store and release electrical charge
according to eqn (59) and (60) (Fig. 12):
V2+ # V3+ + e E ¼ 0.26 V vs. RHE
(59)
VO2+ + H2O # VO2+ + 2H+ + e E ¼ 1.00 V vs. RHE (60)
Flow batteries are inherently exible energy storage systems
because the system power can be scaled by the size of the
electrochemical stack, while the energy storage capacity of the
battery system can be varied with the size of the electrolyte
holding tanks. Hence such ow batteries have many similarities
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to fuel cells. The principal degradation mechanisms for VRFBs
are from the mixing of the vanadium redox couples by vanadium cation crossover between the ow compartments or the
oxidative degradation of the cell components and membrane.
The fact that VRFBs have vanadium based couples at each
electrode [V(II)/V(III) couple at the negative electrode and V(IV)/
V(V) redox couple at the positive electrode] give the ability to
regenerate the electrolytes should crossover occur: the electrolyte volumes in each compartment can simply be mixed and
electrochemically re-activated to regain the capacity of the
battery.374 A primary goal in membrane research for VRFBs is to
limit the vanadium cation diffusion through the membrane,
while maintaining high oxidative stability in the presence of
VO2+ (where vanadium is in the +5 oxidation state) and related
species.375
The VRFB system is being heavily considered for grid-scale
energy storage due to the fast kinetics of the redox reactions and
simple mitigation of electrolyte contamination (due to the allvanadium chemistry). However, there are hurdles to overcome
such as identifying a low-cost source of vanadium (perhaps as a
by-product from metal rening) and sourcing an inexpensive
but high performing membrane for MW-scale installations
(that will require 1000s or even 10 000s of m2 of membrane
material).
The problem with Naon® and aromatic CEMs in RFBs
Naon® and other PFSA IEMs have been deemed currently too
expensive for large-scale grid battery systems where capital cost
is a primary consideration for large-scale stationary electrical
energy storage applications.2a,4d To potentially lower the cost of
these systems compared to PFSA benchmarks, aromatic CEMs
have been explored to good effect in a range of studies.4c Fig. 13
shows a VRFB performance comparison of a sulfonated
aromatic CEM versus a Naon® NRE-212 membrane.376 In this
work, the aromatic backbone was selectively uorinated to
prevent oxidative degradation of the aromatic CEM structure
Fig. 12 Schematic of a VRFB. Typical concentrations for the vanadium
electroactive species (usually sulfate salts) are 1.7 mol dm3 in supporting aqueous H2SO4 (3.3 mol dm3) electrolyte. Dashed arrows
indicate the valance change of the vanadium species during cell
discharge (the positive and negative electrolytes are named after the
discharge reactions), while solid lines represent cell charge processes.
Undesirable vanadium cation crossover can occur, while SO42 anions
can additionally transport through the IEM when an AEM is used
instead of a PEM.
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Energy & Environmental Science
that has been reported for aromatic CEMs exposed to high
oxidation state vanadium.375,377 Because of the distinctly
different ionic domain morphology in the sulfonated aromatic
CEM,378 there is much less vanadium ion crossover in aromatic
IEMs compared to the Naon® benchmark. The lower vanadium crossover mitigates the capacity loss with cycling therefore helping to maintain reasonable ionic conductivity. Similar
to the membrane proton conductivity/methanol permeability
selectivity parameter used in DMFCs,379 the proton conductivity/
vanadium permeability electrochemical selectivity of the
membrane is an important gure of merit for these systems.380
Despite their excellent in situ RFB performance compared to
PFSA membranes, the drawback of aromatic CEMs is that their
oxidative lifetime stability below that of Naon® or that needed
for long-term grid storage applications.
Optimisation of AEMs for low crossover and high conductivity
Because the electroactive vanadium species in VRFBs are positively charged, AEMs with xed cationic groups are attractive
alternatives to CEMs; they have been investigated as ultra-low
vanadium crossover IEMs.381 Additionally, if the vanadium
cations cannot easily penetrate into the membrane, the degradation of aromatic AEMs (on exposure to the vanadium species)
may be mitigated, resulting in a high cell cycle life.382 Typically,
the electroactive vanadium species are dissolved in high
concentrations of aqueous H2SO4 or HCl as the supporting
electrolyte. The charge/discharge reactions of the VRFB in eqn
(59) and (60) can be balanced by H+ diffusion between the
electrode compartments, or correspondingly, the charge can be
balanced by the shuttling of anions across the AEM.
AEMs can display some proton-mediated ionic conductivity
because their transport numbers are not unity for anions.
However, in most AEM-based VRFBs, the majority of the current
will be carried by anions, such as sulphate, traversing the cell.383
Because the anions present will have lower intrinsic mobilities
than protons, high conductivity AEMs and the optimisation of
the conductivity/crossover selectivity ratio is critical in these
systems. A series of benzyltrimethylammonium-containing
Low capacity fade observed in a VRFB using a low-crossover
sulfonated and selectively fluorinated aromatic CEM (filled symbols)
compared to Nafion® NRE-212 (open symbols).
Fig. 13
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Table 4 Properties of select QA-polysulfone AEMs vs. Nafion®-212 in VRFBs
Naon® NRE-212
QA-Radel 2.0
QA-Radel 1.7
IECa/meq. g1
Gravimetric water uptakeb (%)
Ionic conductivityc/mS cm1
VO2+ permeability/m2 s1
0.9
2.0
1.7
28
29
16
44
41
24
3.2 1012
3.7 1014
nm
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a
Measured using 1H NMR. b Liquid water, room temperature. c Equilibrated in aqueous VOSO4 (1.4 mol dm3) + H2SO4 (2.0 mol dm3) solution.
nm ¼ not measureable.
AEMs based on chloromethylated poly(sulfone) were synthesised with different IECs.384 Varying the IEC yielded materials with different conductivity/permeability ratios (Table 4). It
is apparent that an AEM with too low conductivity (e.g. QA-Radel
1.7 from Table 4) will induce high ohmic losses and decrease
the power density of the cell (Fig. 14). On the other hand, too
high a crossover can also negatively impact the power output of
the cell (as is observed with NRE-212 or AEMs with too high an
IEC). The QA-Radel 2.0 AEM had intermediate crossover and
conductivity compared to the other samples and produced the
most power across the set of membranes examined. Similar
observations can be made for thickness optimization of IEMs in
VRFBs where the resistance and crossover must be balanced for
a given set of VRFB conditions.385 Because cell designs and
desired operating points vary across different RFB technologies,
there is no one ideal membrane for all applications. The operating envelope of the cell must be considered in order to arrive
at an optimized membrane conguration.
The study described above demonstrates that AEMs have a
large role to play in VRFB technology. In fact, there are a
number of other VRFB studies showing the utility of AEMs in
these types of energy storage.9f,386 In membrane development
work for RFBs, to date studies have focused on basic descriptions of AEM performance in RFBs in order to codify the
structure–property relationships for these materials in the
unique environment of a redox ow cell. More advanced studies
into the optimisation of the polymer-tethered cationic groups
and membranes with engineered physical structures (to
decrease thickness and increase mechanical strength) are
ongoing to continue to boost the performance of the membrane
in these systems.32a,373b,387
In most AEM RFB studies, high current densities above 200
mA cm2 are still to be demonstrated.388 High current densities
in cells with CEMs are becoming more common with close
attention to the membrane thickness and cell design.389 In
many regards the challenges of direct methanol and other
liquid-fed fuel cell membranes mirror those of VRFBs. Strategies for high conductivity, low crossover membranes are
needed. However, while these issues can be managed, the
largest challenge for new generation VRFB membranes, lifetime, will be harder to overcome. Stationary energy storage
systems will be cycled for tens of thousands of cycles over
thousands of hours of operation. Because most experimental
VRFB membranes are based on aromatic structures, long lifetimes have not been proven to date. For example, post-mortem
spectroscopic analysis of a cardo-poly(ether ketone) AEM used
in a VRFB (100 h of testing) showed a 15% decrease in the QA
3168 | Energy Environ. Sci., 2014, 7, 3135–3191
Fig. 14 Performance optimisation (trade-off between conductivity
and vanadium cation crossover) in a VRFB system. IEMs: circles ¼ QARadel polysulfone AEM (IEC ¼ 1.7 meq. g1), squares ¼ QA-Radel
polysulfone AEM (IEC ¼ 2.0 meq. g1), and triangles ¼ Nafion®
NRE-212.
content.390 The technical and practical challenges for developing stable membranes with proven stability over long periods
of time remain and necessitate further research. As new
experimental AEMs become more available for evaluation in
electrochemical applications (with materials developments
towards thinner membranes with higher conductivities),
further systematic studies on various types of RFBs will
continue to push the application of AEMs in energy storage
devices forward. This will require the clarication and
management of degradation issues.
AEMs in reverse electrodialysis (RED)
cells
Power generation using RED5
Salinity gradient energy (SGE) uses the Gibbs free energy of
mixing of two salt solutions with different salinity to generate
energy (i.e. energy can be extracted where river water ows into
the sea). SGE can also be extracted from industrial processes
where more concentrated salt solutions are generated. SGE is a
non-polluting (no emissions of CO2, SO2 or NOx), sustainable
technology that is available worldwide. The estimated global
energy potential from estuaries only is estimated to be 2.6
TW,391 which is approximately 20% of the worldwide energy
demand.392
Two technologies are available to harvest the energy from the
mixing of two solutions with different salinity:5b,392
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(a) Pressure Retarded Osmosis (PRO)393 uses a semi permeable membrane allowing the transport of water only, while the
solute (salt) is retained;
(b) Reverse electrodialysis (RED – Fig. 15)394 uses IEMs (both
CEMs and AEMs) that selectively transport cations and anions
only. In this electrochemical technology review, the focus is on
RED cell technology.
In RED cells (Fig. 15), a number of CEMs and AEMs are
stacked together in an alternating pattern between an anode
and a cathode with salt water and fresh water owing between
the membranes. Due to the chemical potential difference
between the two solutions, anions are transported through the
AEM and cations diffuse through the CEM (from the seawater to
the river [fresh] water channels). In the electrode compartment,
the ionic charge transport is converted into an electrical charge
transport (electrons) using a reversible redox reaction (rinse
solution), oen based on the redox couple Fe2+/Fe3+; the use of
capacitive electrodes has also been explored.395
Pattle394 was the rst to demonstrate the principle of RED
and Weinstein and Leitz (in the 1970s)396 investigated the effect
of the composition of the salt solutions on the power output.
They stated that large-scale application of RED could become
feasible, but only aer improvements in IEM manufacturing
and optimization of the operating conditions. In the early
1980s, Lacey397 concluded that membranes for RED should be
low cost and have a low electrical [ionic] resistance and a high
selectivity combined with a long service lifetime, acceptable
strength, and dimensional stability. Audinos398 compared the
RED performance of two different types of electrodialysis
membranes (i.e. homogeneous and heterogeneous membranes)
for different salt solutions (NaCl vs. ZnSO4) and obtained a
maximum power output of 400 mW m2. Since the early 2000s,
RED has regained attention as a technology option for
sustainable energy production.399405 Veerman et al.406 determined the power outputs and thermodynamic efficiencies for a
series of six commercially available IEM pairs and used a
Schematic of a reverse electrodialysis (RED) cell. SW ¼ sea
water and FW ¼ fresh water.
Fig. 15
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Energy & Environmental Science
response parameter (the product of these two parameters) to
rank the different membranes; however, this could not related
directly to the properties of the individual membranes.
Most research focuses on generating power from mixing
aqueous NaCl solutions but the effect of using thermolytic
solutions (e.g. ammonium bicarbonate) instead of NaCl has also
been investigated.403 Additionally, the combination of RED with
other technologies (e.g. RED with seawater desalination and
solar ponds,404 RED with reverse osmosis (RO),405 closed-loop
ammonium carbonate RED cells for energy efficient H2 recovery
when combined with an OER anode,407 or RED combined with
microbial fuel cell technology [see later]) have also been reported. Most reports focus on the technological aspects of RED
rather than the membranes. Experiments are usually performed
using commercially available IEMs that have not been specically designed for RED (usually they have a background in
traditional electrodialysis). Such membranes are usually robust,
mechanically strong and consequently rather thick. Moreover,
membrane chemistry, structure, and properties are tailored for
different applications. These can, however, play an essential
role in determining the power output obtainable in a RED cell
(not only in terms of ionic ux but also in relation to membrane
fouling, mixing characteristics, and water composition). As
such, they are key factors determining the net power output
obtainable in a RED cell (discussed below).
Membrane chemistries for RED
Długołe
˛cki et al.408 were rst to systematically investigate
different commercially available membranes for RED applications. They experimentally determined the ionic resistance,
permselectivity, and charge density of a wide range of CEMs and
AEMs, and used these experimental values as input for model
calculations to predict power densities obtainable in RED. Their
main conclusion was that in order to obtain high power
densities, the IEM resistance is critical and should be as low as
possible (preferably area resistances r < 1–2 U cm2), while the
permselectivity is of minor importance (this parameter is
already high at >95%). This was later conrmed experimentally
by a study of Güler et al.409 who measured the power densities
obtainable for a large series of different AEMs and CEMs
(Fig. 16).
RED cells provide an important contrast to the use of AEMs
and CEMs in chemical fuel cells (and other electrochemical
systems containing high and low pH environments) where the
AEMs are typically in alkaline anion form (AAEM) and the CEMs
are in the acidic PEM form: the IEMs in RED cells are commonly
in the Na+ and Cl forms. H+ and OH ions have an additional
conduction mechanism available (the Grotthuss mechanism) so
the conductivities of aqueous solutions containing H+ and OH
ions are signicantly higher than those containing other ions
such as Na+ and Cl (recall Table 1 gives the ion mobilities, m, at
25 C in aqueous solutions). Hence, the IEMs will yield higher
area resistances in RED cells compared to when the same IEMs
are used in chemical fuel cells. This contrast also exists for
microbial electrochemical cells vs. abiotic, chemical, fuel cells
(see later). The situation is even worse for RED cells utilising
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thermolytic solutions as HCO3 ions are less mobile than Cl
ions (this can be mitigated by designing polymers that swell
more in ammonium bicarbonate compared to NaCl
solutions).403
As well as using commercially available membranes, Güler
et al. also, for the rst time, tested tailored made membranes
with chemistries targeted for RED.410 The AEMs and CEMs
synthesised for use in RED cells were systematically investigated
to study the effect of charge density, resistance, and permselectivity in relation to the power output.409,410 Although the
authors observed a reasonable statistical correlation between
the thickness of all (tailor made and commercial) membranes
and the area resistance, no signicant correlations between
both resistance and permselectivity to power output could be
extracted. The results, however, clearly showed that IEM resistance
is more important than permselectivity.
The CEMs synthesized by Güler et al. were based on
sulfonated poly(ether ether ketone) (SPEEK), which is a
common cation exchange polymer frequently used in electromembrane processes. With a degree of sulfonation of 65%, the
SPEEK membranes had an area resistance of 1.22 U cm2 and a
permselectivity of 89%.409 For AEMs, a synthesis using a halogenated polyether [polyepichlorohydrin (PECH)] and DABCO
was employed to simultaneously introduce anion exchange
functionality and crosslinks into the polymer membrane
(during amination);410 poly(acrylonitrile) was also used (as an
inert polymer matrix) to further improve the strength and
stability of the materials. Area resistances of this series of AEM
ranged from 0.82 to 2.05 U cm2 (with permselectivities of 87–
90%). As shown in Fig. 16, this rst attempt to use tailor made
membranes resulted in the highest power output so far, for the
different membranes studied, with a value of 1.27 W m2.409
Previous research showed that the thickness of the membranes
is a critical parameter as well (as it directly inuences the area
resistance). In a non-optimized RED stack (inow limitations
Perspective
dominate the power output), a decrease in membrane thickness
from 130 to 33 mm resulted in an increase in power output of
about 20%; with an optimized stack design, a more pronounced
effect is predicted.410
Geise et al.411 investigated the ionic resistance and permselectivity of a series of synthesized quaternary ammonium PPOand poly(phenylsulfone)-based AEMs. They aimed to develop
structure–property relationships between transport properties
and water content and xed charge concentration that can be
used to assess the membranes with respect to their applicability
in a wide range of electro-membrane processes. It was reported
that the water content of the membranes turned out to be
essential and that the polymers with higher water content tended to have lower ionic resistances and lower permselectivities.
This relationship was not, however, straightforward as it was
highly dependent on the membrane chemistry.
Kwon et al.412 used a nanoporous polycarbonate track-etch
membrane in a parallel structure with nanouidic channels and
a membrane diameter of 10 mm. These membranes are characterized by their low thickness, straight pores, high exibility,
and mechanical stability. The pore sizes investigated were 15,
50, and 100 nm. The authors report that the mechanism for
selective ion transport is based on the formation of a charged
electrical double layer (EDL) on the inner surface of the negatively charged pores. When the EDLs of both pore surfaces
overlap, counter-ions (anions in the case of a positively charged
surface) are preferentially transported while co-ions (cations)
are mostly retained due to electrostatic repulsion. The authors
showed this principle for CEMs, but indicate it should also work
with AEMs. Although the exact type of membrane, material and
concept is not very well addressed in the paper, the mechanism
seems to work best for the smallest pore size (15 nm) and the
power output signicantly decreased on application of
membranes with larger pore sizes. Although not measured in a
real RED stack, the concept was evaluated in a simple two
compartment cell (where only a CEM was used with salt solutions with different salinity on either side); the authors reported
a maximum power of ca. 5 mW (5.8 mW cm2).
Microstructured (proled) membranes
Fig. 16 Experimental power densities obtainable in RED cells for
different membrane couples as a function of the flow rate (number of
cells ¼ 5; spacer thickness ¼ 200 mm).409 Highlighted examples: (a)
University of Twente in-house developed CEM ¼ sulfonated PEEK and
AEM ¼ quaternary ammonium poly(epichlorohydrin); (b) commercially
available Tokuyama CEM ¼ CMX and AEM ¼ AMX; (c) commercially
available Ralex CEM ¼ CMH and AEM ¼ AMH.
3170 | Energy Environ. Sci., 2014, 7, 3135–3191
Usually, the IEMs in a RED stack are separated by non-ionicallyconductive “spacers”, which block part of the membrane area
available for ion transport (the so-called spacer shadow
effect).399,413 To overcome this approach, Długołe
˛cki et al.413
proposed the use of proled IEMs (that contain intrinsic
“spacer” functionality) and recently Vermaas et al.414 (Fig. 17)
and Güler et al.415 demonstrated this concept in practice. These
microstructured IEMs (with integrated spacer functionality)
were developed either by the hot pressing of commercially
available membranes or by solution casting of the previously
described PECH-based AEMs on a structured mould. Although
the boundary layer resistances were higher and the mixing was
poorer inside a stack containing proled IEMs with non-optimised microstructures (compared to a stack using conventional
spacers), higher stack power densities were nonetheless
observed.414 Despite the small improvements observed, proled
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IEMs have a strong future development potential as they lead to
lower pressure drops (loss of power) in the stack and more
optimised structure designs (in terms of mixing and improved
boundary layer resistances) can be envisaged. A very recent
study that has looked into optimising the microstructure design
is the start of efforts to address this.416
Multivalent ions and fouling in RED cells
Although most experiments in laboratory conditions are performed using articial sea and river water (e.g. containing only
NaCl), real world application is different and natural waters
contain signicant amounts (ca. 10%) of multivalent ions
(predominantly MgSO4) and potential foulants such as humic
acids, clay, colloids, and scale inducing minerals. Post et al.417
performed laboratory experiments with feeds containing not
just NaCl but also MgSO4 or MgCl2. A major effect was observed
when the multivalent ions were present where the resistance
increased and the stack voltage decreased. This was attributed
to the transport of these multivalent ions from the dilute solution side to the concentrated solution side (against the
concentration gradient). In a follow-up study, Vermaas et al.418
characterized this transport as “uphill transport”, in accordance
with other membrane processes. They experimentally and
theoretically investigated the effect of increasing the fraction of
MgSO4 (0, 5, 10, 25, 50, and 100%), alongside the NaCl, on the
open circuit voltage and power density with three different
membrane pairs. The presence of MgSO4 in the river water
compartment was shown to have an especially signicant
negative (performance deteriorating) effect. For example, the
presence of only 10% MgSO4 yielded a decrease in stationary
state power output by 29–50% compared to the benchmark
(NaCl only) case. In addition, switching from a NaCl only
solution to a mixture of NaCl and MgSO4, led to voltage
response times in the range of tens of minutes and several
hours (due to ion exchange processes between the membranes
and the feed water). Both researchers concluded that the
Energy & Environmental Science
presence of MgSO4 in natural feed waters is a serious factor that
needs to be taken into account and that the development of
monovalent selective membranes (that preferentially allow the
transport of monovalent ions over multivalent ions) are
required to minimise this undesirable characteristic.
Vermaas et al.419 also performed long term (25 days) experiments with realistic natural feed waters. Before entering the
RED stack, these were ltered through a 20 mm lter (to remove
particulates only). Both a system with IEMs and spacers and a
system with proled IEMs were investigated. The power output
showed a major (40–60%) decrease in only the rst few hours
(Fig. 18). In all cases involving realistic water feeds, deposition
of remnants of diatoms (algae) and clay minerals, organic
fouling, and scaling were observed, depending on the type of
IEM being used. The AEMs were mainly covered with diatoms
and clay minerals, whereas scaling dominated with the CEM.
Although fouling was observed for both proled IEM and
spacer-containing systems, it was less severe with the use of
proled membranes. In addition, recent work has shown that
anti-fouling strategies (e.g. periodic feed water switching or air
sparging) can be very effective in maintaining higher RED cell
power outputs.420
Future perspective
The importance of IEMs that are specically designed for use in
RED cells is evident. Specically tailored IEMs are mandatory if
power outputs (for an economically and commercially viable
process) are to be achieved at the target values of 2–3 W m2
(normalised to membrane geometric area). Modication of
membrane chemistry is a strong tool to allow this goal to be
reached. Focus should be on decreasing the membrane resistance rather than increasing permselectivity. New chemistries
would not only allow the design of membranes with improved
ionic conduction properties, but would also make it possible to
combine this with the additionally desirable development of
monovalent selective (to mitigate against the negative effect of
multivalent species such as MgSO4) and “anti-fouling” (chemical and biological) IEMs.421 Efforts on IEM, including AEM,
development should be focused at these research directions.
AEMs in biological energy systems
Fig. 17 The improved net power output in a RED cell system using
profiled IEMs (solid circles, i.e. an IEM with integrated spacer functionality [see inset SEM micrograph]) in comparison to a standard
system containing non-conductive spacers (open circles).414 SEM:
scale bar ¼ 200 mm, 75 magnification.
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All of the above are chemical, abiotic systems. However, in the
last decade there has been resurgence in the development of
biological fuel cells and related systems. Such systems normally
operate at more neutral pHs compared to most of the abiotic
electrochemical energy systems discussed above, apart from
RED. As with RED, high pH and low pH degradation processes
are less of a concern. Most of the biotic systems that report the
use of AEMs contain a microbial component (Fig. 19).422,423
There are, however, non-microbial biotic systems that have used
AEMs. For example, there has been a notable report of the use of
a polysulfone-AEM in a methanol-fuelled enzymatic fuel cell.7
This system contained a fuel tolerant enzyme-based cathode
(laccase from Rhus vernicera with an enzyme loading of 0.22
mg cm2) along with Pt/Ru-based anodes. This enzymatic fuel
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Fig. 18 Peak power densities as function of the number of days after
the start of the experiment for a stack with spacers (open circles) and a
stack with profiled membranes (filled circles).419
cell demonstrated a very promising power performance (for a
biological fuel cell): 8.5 mW cm2 with a 290 h lifetime.
Microbial fuel cells
Microbial fuel cells424 (MFCs, see Fig. 19a), are generally being
developed for energy efficient treatment of various wastewaters
(containing a variety of substrates [fuels], such as acetate or
sucrose) rather than energy generation per se. They can also be
operated with additional added value functions (e.g. water
soening, NH3 remediation422a,g or electrosynthesis – see later).
MFCs typically contain carbon-based anodes that have been
inoculated with either: (a) a microbial consortia that contain
electroactive microbial species (commonly designated as exoelectrogens and more historically as electricigens) for real world
applications involving a supply of target wastewater (include
human wastewater) that requires treatment; or (b) single
species [monoculture] exoelectrogens (such as Escherichia coli
[E. coli], Shewanella oneidensis, or Geobacter Sulfurreducens) for
more fundamental studies. The exoelectrogens oxidise the
substrates and use the anode as the terminal electron acceptor
when they are located in the target anaerobic (or anoxic) environment of the anode chamber. This avoids the microbes using
dissolved O2 as the terminal electron acceptor (as with normal
microbial respiration), which would be the case if the anode
chamber contains an aerobic environment.
The electrons then pass through an external circuit to the
cathode, which is most commonly an ORR type. The cathode
can be either “air breathing” (supplied directly with air
[passively or actively]) or solution-based (uses O2 [or other
electron accepting species] that is dissolved in a catholyte).
MFCs, therefore operate because the O2 is separated away from
the microbes. The cathodes can contain a variety of catalysts.425
They can be abiotic: e.g. contain Pt-, MnxOy-,422b,426 or
CoTMPP-422p based catalysts or they can even be “non-catalysed”
(i.e. metal-free and carbon-based).427 They can also be biotic: i.e.
either microbial (e.g. containing autotrophic bacteria)422c or
enzymatic (e.g. containing laccase including that excreted from
3172 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
the white-rot fungus Coriolus versicolor).428 Cathodes can also be
photocatalytic (e.g. contain algae or cyanobacteria).429
MFCs can be constructed with a variety congurations
including packed-bed,422d single chamber, or 2 chamber; 3 or
more chambers are oen found with related systems that
involve added-value functions (see later). A large proportion of
these congurations contain an IEM (or other non-ionic and/or
porous separator). One of the earliest reports of a comparison of
2-chambered MFCs containing AEMs vs. CEMs vs. non-ionic
ultraltration membranes was by Logan et al. in 2007.422q This
study discusses the transfer of protons through the AEM via
negatively charged species (phosphate anions) and compared
the use of CMI-7000 CEM and AMI-7001 AEM (both from
Membrane International, USA). A number of key themes,
apparent on review of the literature regarding MFCs containing
AEMs [a small proportion of the total MFC literature], will now
be discussed.
Many studies indicate that MFCs containing (dense) IEMs
have superior performances (e.g. power density outputs‡ and
coulombic efficiencies [CE]) and more stable performances with
time422n when they contain AEMs (as opposed to CEMs). Major
reasons that are put forward for this include:
(a) The lower O2 permeability of the AEMs, especially
compared to Naon® peruorinated CEMs, where reduced O2
crossover from the cathode to the anode is benecial in maintaining an anaerobic environment at the anode (for sustaining
high CEs etc.);
(b) Reduced “pH splitting” effects leading to smaller pH
gradients across the membrane with AEM-based systems: “pH
splitting” is the undesirable lowering of the anode chamber pH
and a raising of the pH at the cathode (common with CEMbased systems);
(c) “Reduced” [or different] membrane fouling characteristics;
(d) Reduced cathode resistances;
(e) Reduced cation-derived precipitates on the cathode
catalyst;
(f) The higher ionic conductivities (lower internal ohmic
resistances§) of the AEMs (vs. CEMs) in MFCs.
The latter point (f) above is an important contrast to the
scenario of AAEM vs. PEMs in (abiotic) chemical fuel cells where
H+ and OH ions are the main conducting ions and where the
OH anions have the lower intrinsic mobilities (Table 1).
‡ Currents and power densities can be normalised to various areas (such as
geometric or “projected” anode, cathode, or membrane areas) and volumes
(such as anode chamber volume or total MFC volume). These are not always
well dened in reported W m2 or W m3 values in the MFC-related literature.
A recommendation is to either unambiguously dene the normalisation being
used or report the absolute currents and powers alongside detailed listing of
relevant areas and volume of the system (and its most relevant components).
The latter will facilitate easier comparisons between the wide diversity of
different systems (congurations) reported in the literature, which currently use
a wide variety of normalisations.
§ Care should be taken when referring to internal resistances, which should be
unambiguously dened. MFC studies oen refer to internal resistances as
meaning the total internal resistances (sum of ohmic + anodic charge transfer +
cathodic charge transfer + contact + interfacial resistances + mass transport
resistances etc.). However, chemical fuel cell (and related devices) studies oen
refer to internal resistances as meaning the internal ohmic resistances.
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Perspective
However, IEMs in MFCs are predominantly in ionic forms that
are not OH and H+; the ions present depend on the nature of
the anolyte and catholyte media and buffers (if present). If you
compare Na+ to Cl, the anion now has the higher mobility.
Early generation (non-phase segregated) AAEMs generally have
higher IECs compared to PEMs to offset the lower mobility OH
ions in APEFCs compared to H+ ions in PEMFCs. The combination of these factors (anion having higher mobility and AEMs
having higher IECs) is why the conductivities of AEMs are now
higher than CEMs when they are compared in MFCs (cf. the
same IEMs used in abiotic fuel cells).
However, a major drawback in the use of AEMs is enhanced
substrate (e.g. acetate) crossover from the anode to the cathode,
which can lead to mixed potentials at the cathode and parasitic
internal currents. If the system in question involves a catholyte
solution (i.e. the O2 is dissolved as part of an aerated electrolyte), then changing this frequently can mitigate against this
substrate crossover effect (if this is realistic in a real world
application?). The loss of metabolites when using an AEM
(especially at low external resistances) can lead to voltage losses
over longer operational periods.422i Membrane deformation
(oen in the opposite direction to that seen with the use of
CEMs) has also been witnessed, which led to an inferior
performance with the use of an AEM;422l this was due to the
trapping of water and gas between the AEM and cathode. This
latter problem can be rectied by using a stainless steel mesh
on the anode side to push the membrane onto the cathode.
The internal resistances§ of MFCs containing dense ionexchange membranes (AEMs and CEMs) can also be higher
compared to MFCs containing porous membranes or with
membrane-less (e.g. single chamber) MFCs, though the lack of
IEM can lead to higher O2 and substrate crossover. For example,
a membrane-free MFC containing a cloth-cathode assembly
(containing a GoreTex® cloth which enhances proton transport
and O2 diffusion to the cathode) had a superior performance
compared to MFCs containing a membrane–cathode assembly
involving either an AEM or a CEM;422m The preparation of MFCs
using the cloth-cathode assembly method was also less time
consuming and is also claimed to be more optimal for scale-up.
Various types of AEMs have been evaluated in MFCs and
these include AMI-7001 (the most commonly encountered),422c,d,f,g,p,q AEMs by Ralex,422b,i,j Tokuyama Neosepta types
(this study compares Neosepta AFN, AM-1, and ACS AEMs and
probes the interface resistances between the AEM and low
buffer [low ionic strength] electrolyte),422k pore-lled type
AEMs,422e and Chinese AEM types by Tianwei422n and Qianqiu
Group (Zhejiang).422a,m
A selected case study is the recent comparison of an in-house
synthesised QA poly(ether ether ketones) [QA-PEEK] AEM to
AMI-7001 in a single chamber MFC.422h The “hydroxide conducting” QA-PEEK AEM ([allegedly] 0.2 mm thick, IEC ¼ 1.39
meq. g1) outperformed the AMI-7001 (450 mm thickness, IEC ¼
1.6 meq. g1, gel polystyrene-divinylbenzene chemistry) with
lower O2 crossover, despite higher substrate crossover; peak
power densities of 60 W m3 and 45 W m3 and CE ¼ 66 and
51%, respectively, were obtained at 30 C. The MFCs contained
PTFE wet-proofed Vulcan XC-72 covered carbon cloth anodes
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Energy & Environmental Science
and carbon-cloth-based air-breathing cathodes that additionally contained a Pt catalyst (loading ¼ 0.5 mgPt cm2 geometric)
and the anodes were inoculated with Anna University domestic
wastewater in a phosphate buffer medium (pH ¼ 7.3–7.6 and
chemical oxygen demand ¼ 400–600 mg dm3). The AMI-7001
visibly degraded aer 250 d of testing, unlike the QA-PEEK AEM.
A second case study422i is where a Ralex AM(H)-PES AEM (QA
polyethylene/polyester based, unspecied thickness and IEC in
the report but normally supplied with < 450 mm dry thickness
and <750 mm fully hydrated thickness) similarly outperformed a
Naon® CEM in comparable at-plate dual-chamber Shewanella putrefaciens (single species) inoculated MFCs at 27 C;
“non-catalysed” graphite plates were used for both the anode
and cathode (maximum voltage and power density of 0.729 V
and 57.8 mW m2 for the AEM vs. 0.676 V and 39.2 mW m2 for
Naon® [power densities normalised to anode geometric
areas]).
A number of the above studies422m,n,q compared the performances of MFCs containing AEMs vs. CEMs.{ This includes the
original study by Logan et al.,422q which compared the performances of 2-chambered MFCs containing either AMI-7001 as an
AEM or CMI-7000 as a CEM (450 mm thickness, IEC ¼ 1.4 meq.
g1, gel polystyrene-divinylbenzene type). A research priority
should be more in-depth comparisons between identical MFC
set-ups containing thinner CEMs and AEMs of identical thicknesses, identical IECs and similar chemistries: i.e. the same
backbone chemistries and just where the cationic (anionexchange) and anionic (cation-exchange) head-groups are the
variable. Aspects that should be studied, including for fundamental investigations, include:
(a) In situ beginning-of-life and longer term performances
(including data on internal resistances [specically including
internal ohmic resistances]§ and current and power outputs
[with clearly dened normalisation]‡);
(b) In situ and ex situ durabilities (i.e. changes in the IEM
chemistry, IEC, conductivity, mechanical stability, and
membrane (bio)fouling with time);
(c) Effects of the different membranes on the nature of the
biolms and microbial consortia in the various zones of the
biotic chambers during in situ MFC testing;
(d) In situ and ex situ studies into the stability and longerterm performances of the electrodes (catalysts) including those
at the cathode (e.g. cathode fouling).
There should also be concerted efforts into the development
of cheap and thin (i.e. low area resistance especially in MFCrelevant anion forms) AEMs that are specically tailored for
MFC-related applications. The AEMs should maintain low O2
permeabilities, exhibit reduced substrate crossovers (to the
cathode), and yield optimised biological outcomes including
reduced performance losses due to biofouling (positively
charged [polycationic] AEMs would selectively inhibit adhesion
or fouling with positively charged bacteria [the opposite to when
{ The properties of the membranes being used are not always unambiguously
reported. As a minimum, we recommend that the following is reported:
thickness (in a well-dened hydration state), IEC, and the chemical nature of
the membrane.
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CEMs are used]). More research aimed at unambiguously conrming the nature of the in situ ion movement through the AEM
(including exactly what ions are involved [OH, PO43, CO32
etc.]) is also justied.
Perspective
Microbial reverse electrodialysis cells (MRC).435 Another
closely related variant is the MRC that was rst reported by
Logan's group in 2011 [Fig. 19c].435e,f An MRC is a combination
of an MFC anode chamber and a RED cell stack that allows for
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Variant systems related to MFCs
Microbial electrolysis cells (MEC).430 MECs are similar to
MFCs but are operated in electrolytic mode (power supplied to
the cell), rather than galvanostatic mode (power generated by
the cell), and are being developed for the energy efficient (low
overpotential) generation of H2 (i.e. that can then be used in
chemical fuel cells etc.). There have been a number of reports on
the use of AEMs in MECs. e.g.431 It has been shown that AEMMECs can outperform CEM-MECs (i.e. improved H2 generation
at a xed applied voltage), oen due to the lower resistances to
ion transport through the AEM. The advantages, disadvantages,
and future challenges regarding the use of AEMs in MECs are
similar to the use of AEMs in MFCs.
Microbial Desalination Cells (MDC).432,433 MDCs are an MFC
variant rst reported in 2009 [Fig. 19b]433n that typically involve a
middle desalination chamber that is separated from the bioanode chamber by an AEM and from the cathode chamber by a
CEM. Some designs also involve a series of desalination
chambers involving AEM and CEM pairs.432,433k A review of the
different types of MDC related systems can be found in ref. 432.
MDCs can use the energy content of the wastewater to help
power the desalination process: i.e. the microbial electricity
generation capacity (or part of it) goes towards desalination.
Total desalination is, however, not possible as the ionic resistance of the desalination chambers would be very high if they
contain totally desalinated water. MDCs are therefore likely to
be applied to water soening applications including soening
prior to a further non-[bio]electrochemical desalination process.433f,m As the AEM tends to be located between the bioanode
chamber and a salt water containing chamber, the AEM-related
advantages, disadvantages, and future challenges are again not
dissimilar to those discussed above for MFCs and RED cells.
MDC investigations are varied and include studies that have
looked into: spatially decoupling the anode and cathode,433c
hydraulically connecting multiple MDCs,433b hydraulically connecting an MDC to an osmotic MFC (MOFCs434 contain a
Forward Osmosis [FO] membrane that can, on their own, allow
desalinated water recovery along with power generation with
superior energy recoveries compared to MFCs containing AEMs
or CEMs),433e scaling up MDCs to litre scale capacities,433h
developing MDC stacks (for faster desalination),432,433k developing hydrid desalination and electrolysis systems for water
desalination and H2 generation,433l developing hybrid microbial
electrolysis desalination and chemical production cells for
desalination as well as acid and alkali production (this type of
system also contains a bipolar membrane between the anode
and chemical production chamber [the AEM separates the
chemical production chamber and the desalination chamber]),432,433i packing MDCs with ion-exchange resins,433a pH
control by using electrolyte circulation,433j and AEM
biofouling.433d
3174 | Energy Environ. Sci., 2014, 7, 3135–3191
Fig. 19 Simple schematics of: (a) a Microbial Fuel Cells [MFC] in air-
breathing single-chamber mode [alternative configuration {not
shown}: a 2-chamber MFC would contain a cathode chamber containing an aerated catholyte and a submerged cathode]; (b) the
simplest configuration of a 3-chamber Microbial Desalination Cell
[MDC]; (c) the simplest configuration of a microbial reverse electrodialysis cell [MRC]. Engineering components, such as gaskets and end
plates, are not shown.
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Perspective
the synergistic enhancement of power production or the energy
efficient production of H2 (the latter when operated as a
microbial reverse electrodialysis electrolysis cell435b,f). An AEM is
located next to the bioanode chamber (à la MFC) and AEMs are
also located as part of the multiple AEM/CEM pairs (as found in
traditional abiotic RED cells where the AEMs separate chambers
with different salinities). As the AEMs are both in contact with
and spatially separated from the biology, the AEM-related
advantages, disadvantages, and future challenges are again not
dissimilar to those discussed above for MFCs and RED cells.
MRC investigations are also varied and have studied topics such
as MRC-based chemical (again acid and alkali) production cells
(the bipolar membrane is located next to the anode chamber)435c
and also systems that involve thermolytic (ammonium bicarbonate) solutions.435d Recently, a “MRC” with only a single cell
“pair” (a single AEM, no CEM and an [NH4][HCO3] catholyte)
has been reported to lead to improved performances compared
to a MRC containing a AEM|CEM|AEM conguration.435a
Microbial electrosynthesis in microbial carbon capture cells
(MCCC).436 Biological conversion of CO2 to fuels and electrofuels is a potentially important clean technology.437 Hence, a
nal bioelectrochemical variant that is worth a quick mention,
and where the application of AEMs may have a future impact, is
for microbial electrosynthesis in MCCCs.436 The example where
an AEM (AMI-7001) has already been applied in an MCCC is
where CO2 is supplied to the cathode (containing a photosynthetic cyanobateria Anabaena sp.) and is sequestered by biological conversion to organic matter.436c Microbial reverseelectrodialysis and electrolysis functions can also be combined
to give a system that produces H2 and sequestrates CO2
(conversion into inorganic carbonates).438
Summary and concluding remarks
Alkaline anion-exchange membranes (AAEMs) are being developed for application in electrochemical devices such as alkaline
polymer electrolyte fuel cells (APEFC). As with the more wellknown proton-exchange membrane fuel cells (PEMFCs),
APEFCs can be operated with Pt-based catalysts. However, it
should be noted that even though the oxygen reduction reaction
is slightly less of a problem at high pHs, the hydrogen oxidation
reaction kinetics is poorer with Pt catalysts in alkali than in
acid. In contrast to PEMFCs, the anode of APEFCs tends to
produce larger performance limitations than the cathode. The
main rationale for the development of devices such as APEFCs
is the promise of the ability to use a broad range of non-precious
metal (non-Pt-group-metals) catalysts.
The conductivities of AAEMs in the OH form can be as high
as H+ conduction in proton-exchange membranes (PEMs such
as Naon®) when the AAEMs are well hydrated, which is the
situation in electrolysers containing polymer electrolytes.
However, the conductivities of AAEMs are much more sensitive
to hydration levels and drop rapidly when exposed to lower
humidity environments (e.g. as found in APEFCs). The development of AAEMs that retain conductivity at lower hydration
levels is a research priority. If OH conductivities are being
measured, it is essential to totally exclude CO2 from each stage
This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science
of the experiment otherwise conductivities will be under estimated (as a mixture of anions [OH/CO32/HCO3] will be
present). To aid in inter-laboratory comparisons, HCO3
conductivities should also be reported.
The biggest research challenge for devices with high pH
environments is to develop chemically stable AAEMs and anionexchange ionomers (AEIs), especially when less than fully
hydrated. Most studies have focussed only on the alkali stabilities (especially of the cationic [anion-exchange] head-groups),
as this is seen as the biggest problem. However, the stability to
in situ generated peroxy species (and resulting radicals) needs to
be explored in much more detail as not much is known to date
regarding AAEMs in APEFCs etc. (in contrast to PEMs in
PEMFCs). Even though a polymer backbone is stable in alkali, it
may not be alkali stable once it is functionalised with cationic
side-groups. Studies of the alkali stabilities of AAEMs and AEIs
must consider the synergistic effects of the cation head-groups
and the backbone. Stability studies should always use spectroscopic evidence alongside secondary evidence such as changes
in ion-exchange capacities with ageing time; the relative
changes in quaternary and non-quaternary exchange capacities
should also be studied.
Despite all of this, AAEMs containing simple (low molecular
weight) quaternary ammonium cationic head-groups (such as
R–N+Me3) may well be stable enough in the OH forms for some
applications if they are kept fully hydrated. This will be especially true for AAEMs that possess phase segregated morphologies, where location of the cationic head-groups in the
hydrophilic channels/clusters will maximise the chances that
they remain suitably hydrated. It is vital that AAEMs/AEIs are
developed with long term stabilities in the OH form at
temperatures >80 C (especially when less that fully hydrated)
for application in APEFCs. Operation at such elevated temperatures (and/or at high current densities) is suspected to lead to
an enhanced or intrinsic tolerance to CO2 in the cathode air
supplies. An alternative approach may be to deliberately direct
the system to utilise CO32 (but not HCO3) conduction as
AAEMs are generally much more stable in the CO32 forms.
The use of alternative fuels in APEFCs is a mixed bag. The
use of alcohols appears not to be viable without the addition of
Na/KOH into the aqueous fuel supplies, which then begs the
question: why use polymer electrolytes in such fuel cells? On the
other hand, the use of non-carbon fuel options may have more
application, especially sodium borohydride for niche (military)
applications and hydrazine hydrate (not anhydrous hydrazine)
for more main-stream markets.
Stability to OH (or CO32) anions is also required if the
AAEMs are to be used in alkaline water electrolysers. However,
such electrolysers have the advantage over APEFCs in that the
AAEMs and AEIs can be kept well hydrated to maximise stability
and conductivity. An aspiration is to develop alkaline polymer
electrolyte electrolysers that generate H2 from pure water (no
added alkali or acid). Electrolysis devices that are aimed at CO2
utilisation are also of increasing interest.
For application in redox ow batteries (RFBs), the anionexchange membranes (AEMs) need to instead be stable to the
redox species present (e.g. vanadium species in all-vanadium
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RFBs [VRFBs]) as well as to low pHs if acidic electrolytes are
present. The hope is that AEMs selectively reject vanadium
cations in VRFBs to reduce vanadium species crossover. A
reduction in water crossover is also considered to be important
with various RFBs and the use of AEMs should be investigated
with regards to this technological requirement.
When applied to electrochemical devices such as reverse
electrodialysis (RED) cells and microbial fuel cells (MFCs), the
alkali stabilities of AEMs are almost irrelevant (as the
membranes are not exposed to high pH environments).
However, the conductivities are lower with ions that are typically
present in these devices (e.g. Cl and not OH). The research
challenge here is to develop thinner membranes (to counteract
the lower conductivities with non-OH anions) but where there
is no reduction in other properties (e.g. no increase in O2
permeability in microbial systems or no lowering of permselectivity or mechanical properties in RED cells). As the AEMs are
in environments with less extreme pHs, then chemistries that
are not stable in alkali (such as imidazolium, phosphonium,
and pyridinium cationic head-groups) may be applicable. This
selection of a wider range of AEM chemistries may well be
advantageous when applied to devices where fouling (including
bio-fouling) may be an issue and needs to be minimised.
It is clear that a single AEM will not be optimal for all
applications and AEMs will need to be specically tailored for
the job at hand. However, there is a signicant chemical and
materials space that can be probed and utilised for the development of tailored AEMs/AEIs.
Statement of author contributions
John Varcoe is lead author who wrote the bulk of the sections on
membrane chemistries and the sections on biological systems
and batteries. He also coordinated all efforts and integrated all
contributions into the manuscript in a consistent manner.
Plamen Atanassov provided the information on fuels that are
alternative to hydrogen for use in fuel cells (such as hydrazine).
Dario Dekel led the discussion (and provided an industrial
perspective) on the application on AAEMs in hydrogen-based
fuel cells. Andrew Herring, Tongwen Xu and Lin Zhuang all
provided signicant contributions on membrane chemistry,
especially the critically important phase segregated systems. Lin
Zhuang also provided input on alternative fuels and electrolysers. Michael Hickner wrote the section on redox ow
batteries. Paul Kohl wrote the section on hybrid anion- and
cation-exchange membrane systems. Anthony Kucernak wrote
the key sections on issues with carbon and carbon-free
supports. He gave some input into various catalyst chemistries.
William Mustain wrote the sections on carbonate cycle fuel cells
and electrolysers. Kitty Nijmeijer wrote the section on Reverse
Electrodialysis Cells. Keith Scott led on the sections on
hydrogen-based electrolysers.
Abbreviations
This lists the subject-specic acronyms used in the main text for
quick reference. This table does not list commonly used
3176 | Energy Environ. Sci., 2014, 7, 3135–3191
Perspective
chemical symbols and nomenclature (e.g. NMR, TEM etc.) that
are well known to the general chemistry audience. Acronyms in
italics are carefully dened in the preamble at the start of the
article.
AAEM
AB
ABCO
ADFFC
AEI
AEM
AEMFC
AFC
AMFC
APEE
APEFC
APM
CCM
CDU
CE
CEM
CNT
DABCO
DAFC
DBHFC
DEFC
DEGFC
DHFC
DMFC
EDL
EG
EOR
FO
HEM
HER
HOR
IEC
IEM
MCCC
MDC
MEA
MEC
MFC
MOFC
MOH
MOR
MRC
ORR
OER
PBI
PECH
PEEK
PEM
PEMFC
PFSA
PGM
PPO
Alkaline anion-exchange membrane
Ammonia borane
1-Azabicyclo[2.2.2]octane (quinuclidine)
Alkaline direct formate fuel cell
Anion-exchange ionomer
Anion-exchange membrane
Anion-exchange membrane fuel cell [h APEFC]
Alkaline fuel cell (taken here as the fuel cell type
containing aqueous K/NaOH electrolytes and not a
AAEM)
Alkaline membrane fuel cell [h APEFC]
Alkaline polymer electrolyte electrolyser
Alkaline polymer electrolyte fuel cell
Alkali-doped poly(benzimidazole)
Catalysed coated membrane
Carbon dioxide utilisation
Coulombic efficiency
Cation-exchange membrane
Carbon nanotube
1,4-Diazabicyclo[2.2.2]octane
Direct alcohol fuel cell
Direct borohydride fuel cell
Direct ethanol fuel cell
Direct ethylene glycol fuel cell
Direct hydrazine fuel cell
Direct methanol fuel cell
Electrical double layer
Ethylene glycol
Ethanol oxidation reaction
Forward osmosis
Hydroxide-exchange membrane
Hydrogen evolution reaction
Hydrogen oxidation reaction
Ion-exchange capacity
Ion-exchange membrane
Microbial carbon capture cells
Microbial desalination cell
Membrane electrode assembly
Microbial electrolysis cell
Microbial fuel cell
Microbial osmotic fuel cell
Metal (normally alkali metal) hydroxide
Methanol oxidation reaction
Microbial reverse electrodialysis cell
Oxygen reduction reaction
Oxygen evolution reaction
Poly(benzimidazole)
Poly(epichlorohydrin)
Poly(ether ether ketone)
Proton-exchange membrane
Proton-exchange membrane fuel cell
Peruoro sulfonic acid
Platinum-group metal
Poly(phenylene oxide)
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Perspective
PRO
PTFE
PVA
QA
QAPS
RED
RFB
RFC
RH
RO
ROMP
RTCFC
SAFC
SGE
SPE
SPEEK
URFC
VRFB
Energy & Environmental Science
Pressure retarded osmosis
Poly(tetrauoroethylene)
Poly(vinyl alcohol)
Quaternary ammonium
Quaternary ammonium poly(sulfone)
Reverse electrodialysis
Redox ow battery
Regenerative fuel cell
Relative humidity
Reverse osmosis
Ring opening metathesis polymerisation
Room temperature carbonate fuel cell
Solid alkaline fuel cell [h APEFC]
Salinity gradient energy
Solid polymer electrolyte
Sulfonated poly(ether ether ketone)
Unitized regenerative fuel cell
Vanadium redox ow battery
Acknowledgements
We wish to thank the following funders and sponsors of the
workshop on “Anion-exchange membranes for energy generation technologies” held at the University of Surrey on 25th –
26th July 2013:1 The UK's Engineering and Physical Sciences
Research Council (EPSRC grants EP/I004882/1, EP/H019480/1
and EP/H025340/1), the University of Surrey's Institute of
Advanced Studies (especially Mirela Dumic for her extensive
assistance),1 the Research Council UK's Energy Programme's
Supergen Hydrogen and Fuel Cell Research Hub (EPSRC grant
EP/J016454/1), Royal Society of Chemistry publishing, Caltest
Instruments Ltd. (UK), Alvatek Ltd. (UK), and Solartron
Analytical UK (Ametek). We also thank the US Army Research
Office for funding from a Multidisciplinary University Research
Initiative
(MURI
contract
W911NF-10-1-0520).
The
following members of the local organising committee are also
thanked: Dr Simon Poynton, Dr Cathryn Hancock, Dr Ai Lien
Ong, Sam Murphy, Sarah Mallinson, Jessica Whitehead and
Lucy Howes.
3
4
5
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Energy Environ. Sci., 2014, 7, 3135–3191 | 3191